in size as the vessels and situated on the upper side of the xylem near the medullary-ray-like strands. Interfascicular wood-prosenchyma is not developed. Wood-parenchyma occurs in groups at the inner margin of the xylem bundles.
The pith is composed of thick-walled cells.
PORTULACEAE.
Portulaca oleracea L. (Leaf only).—Clustered crystals in the leaf small and numerous.
Portulaca quadrifida L. (Leaf and Axis). Clustered crystals in the mesophyll large and few.
Structure of the Leaf:—The epidermis is composed of polygonal cells with outer and inner walls thickened. The outer walls are arched convexly outwards and are granulated. Stomata are more numerous on the lower surface. The guard-cells are elevated and accompanied by subsidiary cells. The front cavity is on a level with the surface. The mesophyll is composed almost wholly of aqueous tissue, the vascular bundles of the veins being surrounded by palisade parenchyma. Internal secretory organs occur neither in the leaf -nor in the axis.
Oxalate of lime occurs in the form of clustered crystals in the aqueous cells of the mesophyll and in the cortex and pith of the axis.
The veins are embedded and are not provided with bundle sheaths; they are surrounded by palisade parenchyma.
The leaf and axis are devoid of hairs.
Structure of the Axis:—The epidermal cells are polygonal with outer and inner walls greatly thickened. The outer walls are convexly arched outwards and are granulated. The stipular rings of silvery long hairs reflect light and protect the axis and leaves against intense light and heat.
The cortex is formed on its outer side of an extensive tissue of thin-walled parenchymatous cells filled with starch granules and on its inner side of chlorophyll containing parenchyma. The cells of the outermost layer of the cortex are collenchymatous. The cortex may occasionally form an aqueous tissue.
The sclerenchymatous pericycle is not developed.
The vascular system is composed of deeply placed vascular bundles separated by thin-walled uniseriate medullary rays. Vessels are small and are arranged in complete rows. The vascular ring is surrounded by green parenchyma. The nearness of the assimilatory tissue to the vascular bundles brings about a quick distribution of the products of assimilation.
(To be continued)
