480 



PRACTICAL BOTANY 



and in general plants which inhabit exposed or excessively 

 dry regions. 



(5) Mesophytes, plants which require a moderate amount 

 of moisture, such as most cultivated plants and our common- 

 est deciduous trees and shrubs. 



Fig. 360. Cross section of stem of pond- 

 weed (Potamogeton) showing air passages 



u, much magnified. After Green 



442. Characteristics of water plants. 



Those water plants which live wholly sub- 

 merged generally differ far more in their 

 form and structure from land plants than do 

 those which (like many grasses and sedges) 

 grow in very wet soil. From the situation in 

 which they grow, submerged plants are less 

 familiar to us than common land plants ; 

 but most people who know out-of-door 

 things well, have seen pondweeds, water 

 crowfoots, bladderworts (Fig. 362), mare's- 

 tail, water weed (^Eloded), or some of the 

 aquatic mosses. It is important to notice 

 how thoroughly most of these are buoyed 

 up and supported by the water. All such plants are soft and 

 limp, without the distinct tough epidermis of land plants, and 

 the leaves are often slender or thread-like. 



Fig. 361. A marsli 

 plant {Limnophila) 



The thread-lilje low- 

 er leaves are water 

 leaves. There are 

 some of an inter- 

 mediate form just 

 above the middle, 

 and the upper ones 

 areairleaves. About 

 one half natural size. 

 After Goebel 



