WATER-PLANTS 49 



and the presence of stomata is rendered unnecessary. 

 For this reason, stomata are either absent on the sub- 

 merged parts, or, if present, they do not open and close. 

 In floating leaves — e.g., water-lilies — normal self -regu- 

 lating stomata occur on the upper surfaces which are in 

 contact with the air. 



3. In submerged aquatics the leaves are thin, and the 

 epidermal cells contain chlorophyll, and so take part in 

 the work of assimilation. This is correlated with the 

 weak light that reaches them under water. By catching 

 it in the outermost cells, the leaves are able to utilize 

 the light at its strongest, before it suffers further loss 

 by penetrating tissues which do not assimilate. 



4. Diminution of the Vascular System. — Since water 

 can be absorbed over almost the whole surface, the presence 

 of a vascular system conveying water from the roots to 

 the leaves is not required, and, like all useless structures, 

 it tends to disappear. The part of the vascular system 

 which conducts water is the wood, or xylem, and it 

 consists of vessels or tubes whose walls have become 

 lignified, or woody. In the oldest aquatics the wood has 

 disappeared entirely, but the phloem — that part of the 

 vascular system which is set apart for the conduction 

 of food-material made in the leaves — remains as it was. 

 The need for the distribution of water throughout the 

 plant has disappeared, but not the need for the distribu- 

 tion of food. Plants growing in flowing water, however, 

 need woody tissue in the stems to enable them to with- 

 stand the strain to which they are subjected. 



5. Roots are also superfluous in aquatics, and tend 

 to disappear. The British plants Wolffla (a duckweed), 

 Utricularia (the bladderwort. Pig. 46), and Ceratophyllum 



' (the hornwort), have no roots. In other cases roots are 

 present, but they do not act as absorbing organs ; they 

 bear no root-hairs, and merely serve to anchor the plants 

 in the mud. In Lemna (duckweed) they still persist, 

 although the plant is free-floating, but the chief purpose 

 they seem to serve is to keep the plant right side up 

 on the surface of the water. 



In dealing with xerophytes, we showed that the leaf 

 is that part of the plant which expresses in the most 

 striking manner the nature of the environment. The 

 same is true in aquatics. 



4 



