ECOLOGICAL GKOUPS 



481 



Definitely stated, some of the most noticeable characteristics 

 often found in submerged aquatic seed plants are 1 : 



(1) Slight development of the root system. 



(2) Slight development of wood cells and vessels. 



(3) Stiffening structures scanty or lacking. 



(4) Air spaces large and abundant. 



(5) The epidermis thin and the cuticle very thin or wanting. 



(6) The leaves often dissected into thread-like divisions. 

 What are probable reasons for character- 

 istics (l)-(5) ? The thread-like divisions of 



the leaves mentioned in (6) are thought to 



favor exchange of gases 



between the water in 



which they are dissolved 



and the leaf interior and 



to offer little resistance 



to currents of water. 



443. Characteristics 

 of xerophytes. There 



are so many types of FlG - ^ 62> A free branch and two buds f 



, . ., a large, common bladderwort 



xerophytes that it is 



... , . f After Beal 



not possible in a few 



words to sum them up or to give the student a comprehensive 

 idea of their peculiarities of structure and of function. Per- 

 haps the easiest way to suggest the leading characteristics of 

 the group is to mention a few familiar representatives of the 

 leading types as follows : 



(1) The olive (Fig. 51), the rubber tree (Ficus elasticci), 

 and the wax plant (Hoya), not uncommon in greenhouses, are 

 good examples of xerophytes with hard, thick-skinned leaves, 

 which have a compact interior parenchyma, without the abun- 

 dant air spaces shown in Fig. 11. 



1 The student must not think of these characteristics as abnormal and 

 of those of ordinary land plants as normal. It is possible that the earliest 

 plants were aquatics and that the aquatics of to-day are more like primitive 

 plants. But it is better to leave such reasoning for more advanced studies. 



