286 THE JOURNAL OF INDIAN BOTANY.
Cucurbitaceae, some Ficoideae, Bubiaceae, some Sola- naceae, Scrophidariaceae, Nyctaginaceae, Amarantaceae, Aristiolochiaceae and Euphorbiaceae,
(d) or of a palisade tissue on both adaxial and abaxial sides in
Gruci/erae, some Capparidaceae, Caryophyllaceae, Elati- neae, some Zygophyllaceae, some Papilionaceae, Caesalpi- neae, some Ficoideae some Composdtae, Salvador aceae, some Asclepiadaceae, some Boraginaceae, some Convolvul- .aceae, some Solanaceae, some Acanthaceae, and some PoZi/- gonaceae,
(e) or of a homogeneous short-celled palisade tissue in Menis-
permaceae, some Capparidaceae, some Sterculiaceae, some
Bhamneae, Bosaceae and some Bubiaceae, if) or of palisade parenchyma round the vascular bundles in
Portidaceae, some Ficoideae, Cyper aceae and Gramincae, {g) or of palisade tissue on the upper side and of arm-palisade
on the lower in most of the species with the exceptions
that have been mentioned. From the various modifications of the assimilatory tissue as described above a process of evolution can be traced. The chlorophyll tissue formed of a homogeneous tissue of polygonal cells in Gentianaceae and Filicinae mark the primary stage from which the various higher stages can be said to have been developed, the assimi- latory tissue formed of a homogeneous palisade tissue forming the final stage of development. The arm-palisade tissue represents an intermediate stage between the spongy tissue and palisade tissue. The modification of tha spongy tissue into palisade tissue through an intermediate stage of the arm-palisade tissue reduces the ventilating system and consequently transpiration, towards which purpose all the adaptations of desert plants are directed.
The tendency of the chlorenchyma to develop into the palisade tissue is further confirmed by the fact that in the axis, the assimila- tory tissue when present, usually consists of arm-palisade cells or of palisade cells.
The arrangement of the palisade tissue in the form of girders round the veins in Gyperaceae and Gramineae is quite adequate in the leaf-blade which is characterised by an abundant articulation and mechanical tissue. The assimilatory tissue in species of these orders performs its own function and affords room for the development of an articulation and mechanical tissue.
In the centric leaves of Zygophyllaceae and Clienopodiaceae, the assimilatory tissue forms more or less a sub-epidermal ring enclosing
