284 THE JOURNAL OF INDIAN BOTANY.
necessary in plants of the nature of Ferns which wither up at the slightest loss of water.
Stomata. — The stomata usually occur on both the surfaces, except when the aqueous tissue is developed along either of the sur- faces. The guard-cells are very thick-walled and vary in their position in different species and sometimes in the same species. They occur
(a) either only on the lower surface in Tiliaceae, Bhamneae, Moring- aceae, some Ficoideae, some Gyperaceae, some Gramineae and Filicinae,
(b) or only on the upper surface in Onagraceae and some Gramineae
(c) or equally numerous on both the surfaces in some Papilionaceae, Caesalpineae, Mimoseae, some Ficoideae, some Compositae, Salvador- aceae, Gentianaceae, Scrophulariaceae and Labiatae, (d) or more numerous on the upper surface in Blepharis and some Gramineae, (e) or more numerous on the lower surface as in most of the species.
The stomata are situated above the assimilatory tissue and when the assimilatory tissue occurs nearer the upper or lower surface or only in the upper or lower half of the mesophyll, the stomata accord- ingly are found on the surface corresponding to that of the assimil- atory tissue. In floating plants, such as Trapa bispinosa the stomata should naturally occur only on the upper surface.
The greater abundance of stomata on the lower surface is due to the more extensively developed ventilating system in the lower half of the mesophyll ; and their equal abundance on both the surfaces is usually due to the bifacial structure of the mesophyll and consequent- ly to the more or less equally developed ventilating system in both the halves of the mesophyll.
The guard-celis may be surrounded by ordinary epidermal cells as in most of the species or are accompanied by subsidiary cells as in Menispcrmaceae, Cruciferae, Vioiaccae, Caryophyllaceae, Portulaceae, Bubiaceae, Salvador aceae, Acanthaceae, Verbenaceae, Labiatae, Com- melinaceae, Gyperaceae and Gramineae. They are situated either
{a) in the same plane as that of surrounding cells, so that the front cavity is placed in a depression formed by outer thickened epidermal walls in Menispcrmaceae, Cruciferae, Capparidaceae, Polygalaceae, Malvaceae, Bhamneae, Lcgu- minosae, Onagraceae, some Ficoideae, some Compositae, Salvador aceae, some Asclepiadaceae, Boraginaceae, Solan- aceae, Acanthaceae, some Amarantaceae, Chenopodiaceae, Aristolochiaceae, Euphorbiaceae, and some Gramineae. {b) or a little below the plane of surrounding cells in Caryo- phyllaceae, Tamariscineae, Zygophyllaceae, Simar ubaceae , Celastraceae and some Compositae.
