ECOLOGICAL GROUPS 



481 



Definitely stated, some of the most noticeable characteristics 

 often found in submerged aquatic seed plants are ^ : 



(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 thovight 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 

 xerophytes that it is 

 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 elastica), 

 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. 



Fig. 362. A free branch and two buds of 



a large, common bladderwort 



After Beal 



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. 



