Open main menu

The Journal of Indian Botany/Volume 1/December 1919/The Physiological Anatomy of the Plants of The Indian Desert

THE

Journal of Indian Botany.

Vol. I. DECEMBER, 1919. No. 4.

THE PHYSIOLOGICAL ANATOMY OF THE

PLANTS OF THE INDIAN DESERT

BY

T. S. Sabnis, B.A., B.Sc.

St, Xavier's College, Bombay.

(Continued from last issue.)

Portulaca quadrifida, L.—{Cont.). The pith is composed of thin-walled cells filled with starch granules.

The two features, viz : the peripheral position of the aqueous tissue and the central position of the assimilatory tissue near the vascular bundles are quite significant. The acqueous tissue from its peripheral position is able to absorb moisture easily and carry on its function without being disturbed by any other tissue situated outside. It further protects the assimilatory tissue from the injurious effects of intense light and heat.

Were the assimilatory tissue peripheral, the products of assimila- tion would have to travel a long distance before they could reach the vascular bundles ; thus the distribution of the products of assimila- tion would be greatly retarded. Therefore either the assimilatory tissue should be centrally placed near the vascular tissue or the aqueous tissue should be reduced, so that the distribution of the assimilatory products may go on rapidly.

It is possible that the assimilatory tissue enclosed by an extensive aqueous tissue may not get enough light, but this cannot be helped. As the development of an aqueous tissue is necessary for succulent plants, the central position for the assimilatory tissue is the only position, so that both the tissues can carry on their work without being interfered with by the other.

TAMARISCINEAE.

Tamarix dioica Boxb — PI. VI, fig. 45. Epidermal cells with inner walls a little more thickened than outer walls. Pits not deep. Cortical parenchyma of small cells. Sclerenchymatous pericycle more or less forming a loose ring of stone-cells with interposing cells thickened and radially striated. Wood forming a composite hollow cylinder. Soft hast forming groups. Medullary rays 2-6 seriate. Pith composed of thick-walled cells.

Tamarix orientalis Forsk.— Fig. 46. Epidermal cells with outer and inner walls thin. Pits quite deep. Cortical parenchyma formed of large thin-walled cells. Sclerenchymatous pericycle more or less isobilateral. Wood forming a composite central cylinder without a central pith tissue. Soft bast forming a continuous ring. Medullary rays uniseriate and few.

Structure of the Axis : — The epidermis consists of thin- walled horizontally tabular cells. The front cavity is depressed and closed above and below by thin walls, fig. 45. The guard-cells are in the plane of or in a plane lower than that of the surrounding cells.

Clothing hairs are not found on the axis of either of the species. External glands, figs. 45, 46, are placed in pits ; they form spherical structures divided by horizontal and vertical three-wall into four thin- walled cells, and are. accompanied on their inner side by two depressed epidermal cells which form the subsidiary cells of the glands. The glands do not project above the surface ; they secrete hygroscopic salts which fill the pits and absorb moisture from the air outside.

The primary cortex is characterised on its outer side by an assi- milatory tissue of palisade cells and on its inner side by cortical colourless parenchyma. There are numerous water-storing tracheids in the cortical parenchyma with pitted or scalariform thickenings, the larger ones being accompanied by a few stone-cells. Cortical parenchyma forms an aqueous tissue and in T. orientalis is composed of large thin-walled cells.

The pericycle is composed of large groups of stone-cells. The stone-cell groups in T. dioica are closely placed all round the soft bast and the cells interposed between them are characterised by sclerosis and by radical striation of the wall. In T. orientalis stone-cell groups are placed on two opposite sides in the form of arcs and the cells interposed between them are thin-walled and parenchymatous. The sides possessing the stone-cell groups may perhaps represent the plane of the axis most affected by the wind ; and the stone-cell groups may have been developed in that plane to protect the axis against violent shaking by the wind. The structure of the wood differs in the two species. In T. orientalis (fig. 46) the wood forms a composite solid central cylinder and is composed of numerous small 'vessels embedded in interfasci- cular wood prosenchyma. In T. dioica (fig. 45) the wood forms a composite hollow central cylinder enclosing a small pith tissue and consists of xylem bundles connected together by interfascicular wood prosenchyma. Medullary rays are 2-6 seriate in T. dioica. In T. orientalis they are uniseriate and few. The soft bast forms a con- tinuous ring in T. orientalis, while in T. dioica it occurs in groups opposite the xylem bundles.

The pith of T. dioica is formed of thick-walled cells, those near the periphery being filled with granules ; it does not occur in T. orientalis.

Internal secretory organs in both the species are represented by some of the cortical cells near the periphery with tanniniferous contents.

ELATINEAE.

Bergia ammanioides Boxb.— PI. VI, figs. 47, 48, 49. A small number of cells with clustered crystals occurring near the veins. Secre- tory cells with tanniniferous contents found in the pith. Clothing hairs not found on the leaf. Hairs on the axis in the form of very thin-walled uniseriate trichomes. Glandular hairs absent on the leaf and axis. T. S. of the axis quadrangular. Assimilatory tissue in the axis formed of chloroqhyll containing parenchyma. Sclerenchymatous pericycle in the form of small stone-cells at the angles. Medullary rays absent.

Bergia odorata Edgeto.— PI. VI, fig. 50 ; PI. VIII, figs. 51, 52, A layer of polygonal cells with clustered crystalsin the middle of the mesophyll. Numerous clustered crystals near the veins. Secretory cells with tanniniferous contents in the cortical parenchyma, soft bast and pith. Ordinary unicellular hairs and uniseriate trichomes found on the leaf and axis. Glandular shaggy hairs occurring on the leaf and axis. T. S. of the axis circular. Assimilatory tissue in the axis composed of short palisade cells. Sclerenchymatous pericycle forming a loose ring of small groups of stone-cells. Medullary rays ]-2 seriate.

Structure of the Leaf: — The epidermis is composed mostly of horizontally tabular cells with very large water-storing cells inter- calated amongst them. The water-storing cells are sometimes divided by cross walls into unequal halves, the lower half being much larger than the upper one. The inner walls are thin and arched convexly inwards, so as to come into close contact with the assimilatory tissue below. The lateral walls are thin and straight ; the outer walls are flati and greatly thickened. Stomata occur on both the surfaces, being a little more numerous on the lower. Guard-cells are elevated above the plane of the sur- rounding cells and the front cavity is a little raised above the surface. Stomata are surrounded by 3-4 ordinary epidermal cells.

The mesophyll is composed of short palisade cells on either side without a middle tissue except for a layer of polygonal cells contain- ing clustered crystals in the middle of the mesophyll in B. odorata.

Internal secretory cells are not found in the leaf. Oxalate of lime occurs in the form of numerous clustered crystals near the veins. In B. odorata there is a layer of polygonal cells with clustered cry- stals in the middle of the mesophyll (fig. 51). In the axis clustered crystals occur in the cortical parenchyma and pith. Veins are provided with sheaths of green thick-walled cells. Larger veins are vertically transcurrent above and below by colourless parenchyma.

Hairy covering on the leaf and axis consists of clothing and glandular hairs. Clothing hairs are partly unicellular and partly uniseriate trichomes. They are not found on the leaf of B. ammani- oides and those on the axis are very thin-walled. Glandular hairs occur only in B. odorata and are of a shaggy type ; they are composed of a multicellular stalk and of a multicellular head (figs. 51, 52).

Structure of the Axis : — The epidermis is two-layered. Outer epidermal cells have outer walls greatly thickened, the lateral walls being thin. The stomata are like those on the leaf and are surround- ed by 4-5 ordinary epidermal cells. The primary cortex is charac- terised in its outer portion by an assimilatory tissue, which in B. odorata (fig. 52) is composed of short palisade cells and in B. ammani- oides (fig. 66) of chlorenchyma. Inner portion of the cortex is formed of large-celled cortical parenchyma.

The wood in both the species presents a quadrangular appearance. It is broader at the angles and is much narrowed between. Inter- fascicular wood prosenchyma is extensive. The medullary rays occur in B. odorata and are 1-2 seriate. The vessels are arranged in rows. Wood parenchyma is scantly developed.

The pith is composed of thin-walled cells. Internal secretory organs are represented by secretory cells with tanniniferous contents. They occur in the cortical parenchyma in the soft bast and pith of B. odorata and only in the pith of B. ammanioides.

MALVACEAE

Sida grewioides Guill. — Figs. 53, 54. Woody. Stomata more numerous on the lower surface. Internal glands numerous in the mesophyll. Clustered crystals occurring near the veins of the leaf and in the soft bast of the axis. Clothing hairs tufted and more numerous on the lower surface. Glandular hairs pitcher-shaped and more numerous on the lower surface. Cortex characterised by cork and collenchyma. Assimilatory tissue formed of chlorenchyma. Bast fibres occurring in the soft bast. Wood uniformly broad. Medul- lary rays uniseriate. Pith formed of thin-walled cells.

Abutilon fruticosum Quill. — Figs. 55, 56. Woody. Stomata numerous on both sides. Internal glands absent. Clustered crystals found near the veins of the leaf and in the collenchyma, soft bast and pith of the axis. Clothing hairs tufted and equally numerous on both sides. External glands club-shaped and equally numerous on both the sides. Cortex characterised by cork and collenchyma. Wood uniformly broad. Vascular bundles occurring in the pith. Medullary rays 1-3 seriate. Pith formed of thick-walled cells.

Pavonia arabica Steud. — Woody. Stomata more numerous on the lower surface. Secretory cavities numerous in the mesophyll. Clustered crystals abundant in the leaf and axis. Clothing hairs tufted and more numerous on the lower surface. Tufted hairs along with simple thick-walled unicellular hairs found on the axis. Assimi- latory tissue formed of chlorenchyma. Bast fibres occurring in the soft bast. Wood reduced on the lower side of the inclined axis. Medullary rays uniseriate. Pith formed of thin-walled cells.

Hibiscus micranthus Lf— PI. VIII, figs. 57, 58. Woody. Stomata more numerous on the lower surface. Internal glands absent in the leaf and axis. Clustered crystals found near the veins in the leaf, and in the collenchyma and pith of the axis. Clothing hairs tufted. External glands club-shaped and few on the leaf and axis. Assimila- tory tissue formed of palisade cells. Cortex characterised by collen- chyma. Sclerenchymatous pericycle and wood reduced on the lower side of the inclined axis. Medullary rays 1-3 seriate. Pith formed of thin-walled cells.

Gossypium herbaceum L— Figs. 59, 60. Woody. Stomata more numerous on the lower surface. Internal glands in the leaf and axis in the form secretory cells and secretory cavities. Some of the pith cells holding tanniniferous contents. Clothing hairs tufted. Glandular hairs spherical and more numerous on the upper surface of the leaf and numerous on the axis. Cork subepidermal. Scleren- chymatous pericycle and wood reduced on the lower side of the inclined axis. Medullary rays 1-3 seriate. Pith formed of thin-walled cells.

Structure of the Leaf : — Epidermal cells are tabular with outer- walls thickened and papillose. Lateral walls are straight. The sur- face of the leaf in Abutilon fruticosum is characterised by ridges and furrows. There are cells of considerable dimensions with waterstorage function intercalated amongst the ordinary epidermal cells in Hibiscus micranthus (fig. 57A). They are present on both the sides and have their inner walls convexly arched inwards so as to come into close contact with the assimilatory tissue.

The stomata are more numerous on the lower surface. The front cavity is placed in a depression formed by the outer thickened and papillose walls. The guard-cells are in the plane of the surround- ing cells as in Abutilon fruticosum and Sida Qrexoioid.es (fig. 53), or they are elevated as in other species. The elevated position of the guard-cells can be accounted for by the occurrence of a dense covering of tufted hairs. The mesophyll is composed of palisade tissue on the adaxial side and of arm-palisade tissue on the abaxial side. The palisade tissue in Sida greioioides is formed of compact cylindrical groups. The mesophyll is characterised by the abundance of internal glands in Sida grewioides, Pavonia arabica and Gossypium herbaceum.

The internal secretory organs in Gossypium herbaceum are re- presented partly by some of the palisade cells and a layer of polygonal cells above the arm-palisade tissue with tanniniferous contents and partly by mucilaginous secretory cavities with a lining layer of cells, and situated in the middle of the mosophyll and in the arm-palisade tissue. In Pavonia arabica, there are mucilaginous secretory cavities situated in the middle of the mesophyll below the vascular bundles of the veins. In Sida greioioides (fig. 53), there are groups of loosely arranged palisade-like cells, faintly green in colour and placed between groups of palisade cells ; there are also rounded or elliptical structures amongst the arm-palisade cells, also faintly green in colour. These structures in the mesophyll are either schizogenously formed internal secretory cavities or water-storage cells.

Oxalate lime occurs in the form of clustered crystals near the veins. In the axis clustered crystals occur in the cortex and pith of all species except Pavonia arabica. In Sida grewioides and Pavonia arabica numerous small-clustered crystals occur in the soft bast.

The veins are embedded except some of the larger veins in Gossypium herbaceum which are vertically transcurrent above and below partly by means of sclerenchyma and mostly by collenchyma. The veins are provided with bundle -sheaths of green thin-walled cells.

The hairy covering consists of densely placed tufted hairs. The rays are unicellular and are sunk directly in the epidermis, so that the hairs seem to be formed by a group of epidermal cells, fig. 55. The rays on the lower portion of the mid-rib are borne on a short multicellular stalk. The hairy covering is denser on the lower surface of the leaf. Glandular hairs in Abutilon fruticosum, Pavonia arabica and Hibiscus micranthus (fig. '57) are club-shaped and are composed of a basal stalk-cell and of a head divided by horizontal and vertical walls. Sida greioioides (fig. 54) possesses pitcher-shaped uniseriate glandular hairs. External glands in Gossypium herbaceum (fig. 59) are spherical and are composed of a basal stalk-cell and of a head irregularly divided. External glands are numerous on both the surfaces of the leaf and are placed in depressions of the epidermis in all members except Abutilon fruticosum. They are more numerous on the lower surface in Sida grewioides and on the upper surface in Gossypium herbaceum.

Structure of the Axis : — The epidermis consists of small tabular cells with outer walls thickened and papillose. The lateral walls are straight. The hairy covering is composed of tufted hairs as described already. Besides the tufted hairs there are thick-walled unicellular hairs, resembling the rays of the tufted hairs, in Pavonia arabica. External glands occur on young branches and have the same charac- ters as of those on the leaf.

The primary cortex is characterised by subepidermal cork in Sida grewioides, Abutilon fruticosum (fig. 56) and Gossypium herba- ceum (fig. 60). Collenchyma occurs in the cortex in all members ; it may form long strands as in Pavonia arabica or a continuous ring as in other species. Assimilatory tissue consists of palisade cells in Hibiscus micranthus (fig. 54) ; in other species it is formed of chlorenchyma.

The pericycle is composed of closely placed rhomboidal groups of stone-cells ; it is reduced on the lower side of the inclined branches of Hibiscus micranthus. There are numerous small groups of bast fibres in the soft bast of Sida grewioides and Pavonia arabica.

The wood forms a composite hollow cylinder in all members. The vessels are small and arranged in closely placed rows. The interfascicular wood prosenchyma is not very extensive. The medull- ary rays are uniseriate in Sida greioioides and Pavonia arabica ; in others they are 1-3 seriate. In Abutilon fruticosum there occur vascular bundles in the pith close to the xylem cylinder. On a small portion of the axis in Pavonia arabica, Hibiscus micranthus and Gossypium herbaceum the wood is reduced ; in this portion of the axis the wood is much narrowed and vessels are few and small. This may be accounted for by the inclined nature of the axis, the wood being reduced on the lower side.

The pith consists of thick-walled cells in Abutilon fruticosum ; it is composed of thin-walled cells in others.

Internal glands occur in the cortex in the form of secretory cavities with a lining layer of cells and with pinkish contents in Gossypium herbaceum. Oxalate of lime is found in the form clustered crystals in the soft bast in Sida greiuioides and Gossypium herbaceum (fig. 60) in the cork, eollenchyma, soft bast and pith of Abutilon fruti- cosum (fig. 56) and in the eollenchyma of Hibiscus micranthus (fig. 58). Oxalate of lime does not occur in any form in Pavonia arabica.

Anamolous structures are represented by vascular bundles in the pith of Abutilon fruticosum as already mentioned.

General Review: — Epidermal cells are tabular with outer walls not much thickened. Large water-storing cells are intercalated amongst the ordinary epidermal cells in Hibiscus micranthus (fig. 57). Guard- cells are usually a little elevated. The front cavity is depressed. A dense covering of tufted hairs occurs on the leaves and young bran- ches. External glands are either club-shaped (tig. 57), pitcher-shaped (fig, 54) or spherical (fig. 59) ; they are usually placed in epidermal depression.

The mesophyll is composed of palisade tissue on the adaxial side and of arm-palisade tissue on the abaxial side. Internal secretory organs occur in the leaf and axis in the form of secretory cells or secretory cavities. The veins are embedded except the larger ones in Abutilon fruticosum ; they are provided with bundle-sheaths. Oxalate of lime is found in the form of clustered crystals in the leaf and axis, The assimilatory tissue in the axis is either composed of palisade tissue or of chlorenchyma. The cortex is usually strengthened by eollenchyma. The cork is subepidermal. The pericycle is formed of rhomboidal groups of stone-cells. Numerous small groups of bast fibres are found in the soft 'bast of Sida greiuioides and Pavonia arabica* The wood forms a composite hollow cylinder. The vessels are small and few. Interfascicular wood prosenchyma is not ex- tensive. Medullary rays are 1-3 seriate. Vascular bundles occur in the pith of Abutilon fruticosum. The pith is formed of thin-walled cells.

STERCULIACEAE.

Melhania Denhamii Br. Pigs. 61, 62. Lower surface of leaf deeply furrowed. Mesophyll formed of short palisade cells on the adaxial side and of arm-palisade ones on the abaxial side. Upper epidermal cells with tanniniferous contents. Numerous mucilage canals in the pith. Solitary crystals in the leaf and axis. Clothing hairs tufted. Glandular hairs club-shaped. Pericycle formed of an outer loose ring of stone-cells and of an inner loose ring of bast fibres. Vessels large. Medullary rays 1-3 seriate and broadening outwards in the form of wedges between the groups of soft bast. Melhania magnifolia, Blatfc. and Hall.

Lower surface of the leaf with furrows not deep. Mesophyll formed of palisade tissue on the adaxial side and of an arm-palisade tissue on the abaxial side. Numerous cells with tanniniferous con- tents near the veins. Solitary crystals occurring below the upper epidermis and near the veins of the leaf. Clothing hairs tufted. Glandular hairs club-shaped. Pericycle formed of a single loose ring of stone-cells. Vessels small and few. Medullary rays uni- seriate. Mucilage canals in the pith few.

Structure of the Leaf : — Epidermal cells are tabular, with outer walls a little thickened and convexly arched outwards. Lateral walls are straight. The lower surface is characterised by furrows which are much deeper in ilf. Denhamii. Stomata are more numerous on the lower surface and occur in the furrows ; they are surrounded by ordinary epidermal cells. Guard-cells are elevated and the front cavity is on a level with the surface. The mesophyll in M. Denhamii is composed of a homogeneous palisade tissue ; in M. magnifolia there is a palisade tissue on the adaxial side and arm-palisade tissue on the abaxial side.

Internal glands are represented in the axis by cells with mucil- aginous membranes in the cortical parenchyma and by numerous mucilage canals of schizogenous origin in the pith. In the leaf of M. Denhamii upper epidermal cells and numerous polygonal cells near the veins hold tanniniferous contents. Oxalate of lime occur in M. Denhamii in the form of solitary crystals near the veins of the leaf and in the cortical parenchyma and pith of the axis. In M. magni- folia bundles of solitary crystals occur near the veins and in a layer of tabular cells below the upper epidermis.

The veins are enclosed in green bundle-sheaths and are vertically transcurrent above by colourless parenchyma. The veins of the mid- rib are vertically transcurrent above by clourless thick- walled paren- chyma and below by collenchyma.

Hairy covering on the leaf and axis consists of densely placed tufted hairs which are more numerous on the lower surface. The rays are unicellular and thick-walled and are sunk directly in the epider- mis, so that the hairs seem to be formed by a group of epidermal cells (fig. 61). The rays on the lower surface of the mid-rib and on the axis are placed on a short multicellular stalk (fig. 62). The glandular hairs on the leaf and axis are club-shaped and are composed of a stalk- cell and a head divided by horizontal and vertical walls (fig. 62). The external glands are more on the upper surface and protect the palisade tissue against the strong light and glare by means of their secretions. 1480—14 Structure of the Axis .'—The epidermal cells are small and poly- gonal, with outer walls greatly thickened and convexly arched outwards. The cortex is characterised by subepidermal cork and by collenchyma. The assimilatory tissue is formed of chlorenchyma.

The pericycle in M. Denhamii is formed of an outer ring of groups of stone-cells and of an inner ring of radially elongated groups of bast fibres. The groups of stone-cells and of bast fibres are separated by colourless parenchyma. In M. magnifolia the pericycle is formed of a loose ring of groups of stone-cells.

The wood is composite and is much narrowed at two opposite points which probably represent the plane at right angles to that which is affected by the prevailing wind. The vessels in the broader portion are larger and less numerous. The wood in M. magnifolia is compo- site and is of uniform breadth. Interfascicular wood prosenchyma is extensive and is composed of cells with thick-walls and small lumina. Medullary rays in ill. Denhamii are 1-3 seriate, broadening outwards in the form of wedges between the groups of soft bast and are in contact with parenchymatous cells separating the groups of stone-cells and of bast fibres. Medullary rays in M. magnifolia are uniseriate.

The pith is distinguished by the occurrence of mucilage canals and is composed of thin-walled cells.

TILIACEAE.

Grewia populifolia Vahl.— Front cavity placed in depres- sions formed by the thickened outer epidermal walls, Solitary crystals few and occurring near the veins and in the cortex. Veins vertically transcurrent above and below by sclerenchyma. Clothing hairs tufted. Glandular hairs club-shaped with the head divided by horizontal and vertical walls. Pericycle forming a composite ring of stone-cells. Medullary rays uniseriate.

Grewia villosa Willd. — Front cavity on a level wifch the surface. Solitary crystals numerous and occurring near the veins, in the cortex and pith. A few conglomerate crystals present near the veins. Veins vertically transcurrent above by sclerenchyma 'and below by bundle-sheath cells. Clothing hairs tufted. Glandular hairs club- shaped with the head divided by horizontal and vertical walls. Cork subepidermal with one or two layers of thickened and lignified cork- cells at its lower end. Pericycle formed of a composite ring of' stone-cells. Medullary rays 1-2 seriate.

Grewia abutilifolia Vent.— Figs. 63, 64. Front cavity on a level with the surface. Solitary crystals few and occurring near the veins and in the cortex. Veins vertically transcurrent above and below by sclerenchyma with water-storage tracheid-like structures at their terminations. Clothing hairs tufted and unicellular. Glandular hairs club-shaped with the head divided by horizontal and vertical walls. Pericycle of stone-cells and not composite. Medullary rays 1-3 seriate.

Corchorus trilocularis L.— Fig. 65. Clustered crystals near the veins and few. Glandular hairs club-shaped with the head divided by horizontal and vertical walls. Clothing hairs unicellular* Epidermis of the axis with outer walls considerably thickened and cuticularised. Lateral walls thickened. Wood narrowed on the lower portion of the inclined axis. Medullary rays uniseriate.

Corchorus antichorus Baens.— Clustered crystals near the veins and few. Clothing hairs unicellular. Glandular hairs club- shaped with the head divided by horizontal and vertical walls. Epi- dermis of the axis with the outer walls greatly thickened and cuticularised. Lateral and inner walls also a little thickened. Medull- ary rays 1-2 seriate. ■

Corchorus tridens L.— Figs. 66, 67. Clustered crystals near the veins and numerous. Clothing hairs unicellular. Glandular hairs club-shaped and with the head divided by horizontal walls* Epidermis of the axis with outer walls greatly thickened and with lateral and inner walls also a little thickened. Medullary rays 1-2 seriate.

Structure of the Leaf : — The epidermis of the upper side con- sists of almost cubical cells and of the lower side of horizontally tabular cells. The outer walls are a little thickened ; the inner and lateral walls are thin. The lateral walls are straight. Epidermal C8lls surrounding the sfcomata are usually much smaller and are of the nature of subsidiary cells.

The stomata are more numerous on the lower surface, while in species of Greiuia they occur only on the lower surface. The guard- cells are elevated and the front cavity is on a level with the surface. In species of Corchorus epidermal cells on either side of the guard- cells are elevated and are much smaller than the ordinary epidermal cells ; they seem to be of the nature of subsidiary-cells. The stomata on the axis are like those on the leaf.

The mesophyll is composed of palisade tissue on the upper side and of arm-palisade tissue on the lower.

Oxalate of lime occurs in the form of numerous clustered crystals near the veins in species of Corchorus (fig. 66). In species of Greivia solitary crystals are found near the veins (fig. 63), and in the cortex and pith. Solitary crystals are sometimes found in groups of 3-4 in the cells ; they are sometimes also aggregated in the form of conglomerate crystals in the axis of species of Grcwia.

The veins are embedded and are not provided with bundle- sheaths in species of Gor chorus. The veins are vertically transcurrent above and below by sclerenchyma in Grcwia poimlifolia and Grcwia abutilifolia and above by sclerenchyma and below by sheath cells in Grewia villosa. The veins in species of Grcwia are provided with bundle-sheaths. In Grcwia abutilifolia there are structures of the nature of water-storing tracheids at the terminations of the veins.

Hairy covering on the leaf and axis consists of clothing and glandular hairs. The clothing hairs in species of Corchorus are unicellular, thick- walled and not dense (fig. 6G). In Grcwia they are tufted (figs. 63, 64), the constituent rays being unicellular and thick- walled. Besides the tufted hairs, there are unicellular thick-walled hairs, resembling the rays of the tufted hairs, on the axis of Grewia abutilifolia (fig. 63). The glandular hairs are club-shaped and are composed of a stalk-cell and of a head divided only by horizontal wall3 (figs. 65, 63), or by both horizontal and vertical walls (tig. 64).

Structure of the Axis : — The epidermis in species of Corchorus consists of horizontally tabular ceils with outer walls thickened and arched convexly outwards, the thickening: of the outer walls being quite considerable in Corchorus trilocularis and Corchorus tridens. The lateral and inner walls are also thickened. The epidermal cells in species of Grcwia are horizontally tabular and are uniformly thickened on all sides. The lateral walls are straight in members of both the genera.

The cortex is characterised by subepidermal cork in species of Grcwia, the innermost layers of which in G. villosa are composed of thickened and lignified cork-cells. Cork does not occur in species of Corchorus. Three tissues may be distinguished in the cortex : in species of Corchorus outermost parenchyma, middle collenchyma and innermost parenchyma ; in species of Grewia outermost cork, middle collenchyma and innermost parenchyma.

Internal secretory organs are represented in the axis of members of both the genera by mucilage cavities in the cortical parenchyma (figs. 67, 64) and in the pith (fig. 64).

The pericycle is composed of closely placed groups of stone-cells with very small lumina (figs. 64, 67). The wood forms a composite hollow cylinder. The vessels are arranged in rows. The interfascicular wood prosenchyma is extensive and is composed of cells with wide lumina. The medullary rays are numerous ; they are uniseriate in Q. trilocularis, G. vopulifolia and G. villosa, 1-2 seriate in C. anti-chorus and C. tridens and 1-3 seriate in G. abutilifolia. Wood parenchyma is poorly developed.

The wood in G. trilocularis is narrowed on one side, the narrowed portion being characterised by a larger number of vessels and the broader portion by more extensive wood prosenchyma. These modifications in the structure of the wood may be the result of the inclined nature of the axis, the narrowed portion being situated on the lower side of the inclined axis. The abundance of wood pro- senchyma on the upper side prevents the axis from bending.

The pith is composed of thin-walled cells in species of Corchorus and of thick-walled cells in species of Grewia.

General Review : — There are a number of characters which can be used for the diagnosis of the two genera.

Corchorus : — Stomata with smaller epidermal cells on either side of the guard-cells. Oxalate of lime in the form of clustered crystals near the veins. Veins embedded and not provided with bundle-sheaths. Clothing hairs unicellular. Outer walls of epider- mal cells of the axis greatly thickened and cuticularised. Cortex composed of three zones — outermost parenchyma, middle collenchyma and innermost parenchyma. Pith composed of thin-walled cells.

Grewia : — Stomata with ordinary epidermal cells on either side of the guard-cells. Oxalate of lime in the form of solitary crystals. Veins vertically transcurrent and provided with green bundle-sheaths. Clothing hairs usually tufted. Epidermal cells of the axis uniformly thickened on all sides. Cortex composed of three zones — outermost cork, middle collenchyma, innermost parenchyma. Pith formed of thick-walled cells.

{To be continued.)

Page:The Journal of Indian Botany.djvu/140 Page:The Journal of Indian Botany.djvu/142