INTRODUCTION 25 



they may be thoroughly oxygenated by the aid of the 

 air in the water. This is their chief difficulty, to 

 obtain air. The oxygen liberated in carbon assimi- 

 lation is therefore largely retained for respiration, 

 and to render the plant buoyant, so that it may 

 struggle to keep its place in the sun. 



The cuticle is thin and water is absorbed at all 

 points. There is no need for stomata or guard-cells. 

 The air spaces are always open. The water-lilies 

 and other floating-leaved plants have the stomata on 

 the upper surface, whereas land plants have them 

 below or above in some cases, or both. 



The leaves are thin and the chloroplasts are in the 

 epidermal cells, in order to reach the less accessible 

 light more readily. There is no need for strengthen- 

 ing tissues, as the surrounding water gives support, 

 so that vascular bundles are not highly developed, 

 and xylem is not present in many cases, but phloem 

 is general. Many are monocotyledons. 



The roots are reduced and serve as organs of 

 attachment, without root-hairs. 



The submerged leaves are much divided to present 

 a greater surface to the light, and to facilitate the 

 absorption of carbonic acid gas and oxygen in the 

 water, and to offer less resistance to the current. 

 Current leaves are often developed in the floating 

 types till the floating leaves are developed. Ribbon 

 leaves are adapted to the current. Awl-shaped leaves 

 are found in floating or riparial types. 



The floating leaves are circular or nearly so, with 



