LAND- AND WATER-FORMS 177 



The leaves of the Starwort do not differ appreciably in the 

 two forms, since the blades are thin and broad in both, but 

 there is a distinct cuticle and a better developed vascular system 

 in that of the land-plant. Since stomata are in this case present 

 even on (the upper sides of) the submerged leaves, the only 

 difference with respect to them is that they remain closed 

 in the water-form. A marked contrast is, however, pre- 

 sented by the leaves of the two forms of the common Water 

 Buttercup (Ranunculus aquatilis, Fig. 94). Those of the land- 

 form (L) have a definite palisade layer, which occupies the bulk 

 of the mesophyll, and the epidermis is devoid of chloroplasts 

 and provided with stomata through which gaseous exchange 



FIG. 94. Transverse sections of the leaves of the land (L) and water (W) 

 forms of the Water Buttercup (Ranunculus ajuatilis). 



takes place. In the leaves of the water-form (W), on the other 

 hand, palisade tissue is absent and there are large and con- 

 spicuous intercellular spaces. There are no stomata in the 

 epidermis whose cells contain chloroplasts. Similar differences 

 can be observed between the floating and submerged leaves of 

 this plant, but the contrast is more striking in the Mare's-tail 

 (Hippuris), where the leaves borne above and below water are 

 of the same form. 



A combination of aquatic characteristics with others, usually 

 encountered in the vegetation of dry habitats, is not uncommonly 

 exhibited by plants rooted in boggy ground, but whose shoots 

 are exposed to conditions tending to encourage excessive trans- 

 piration. These conditions may in part explain the phenomenon, 

 12 



