CHAPTER XV 

 GROWTH HABITS OF PLANTS 



Various attempts have been made from time to time to classify 

 plants into life-form groups and to interpret these groups in terms of 

 the environment. These classifications have been founded, for the 

 most part, on the role played by a particular species in vegetation 

 and its life history under the conditions prevailing in its habitat, 

 with special reference to duration, propagation and protection dur- 

 ing unfavorable seasons. These classifications have not proven so 

 useful as their authors hoped, largely, perhaps, because we still are 

 unable to interpret ecologically many of the growth-forms that we 

 find, but such classifications are of considerable value in certain 

 types of ecological work. 



The simplest classification of this sort is the one we have already 

 given; namely, the classification into xerophytes, mesophytes, and 

 hydrophytes, although it is based only on a single relation, that of 

 water supply. In this chapter we shall first discuss some of the char- 

 acteristics of these three general types of plants and then consider 

 briefly two other classifications that have been used. 



107. Plants of Hydric Habitats.— The habitat of a plant consists 

 of the sum total of the environmental conditions under which it 

 lives. It is the place of abode of the plant together with all the 

 environmental factors that are operative within the abode. Hydric 

 habitats are those occupied by hydric plants (Fig. 73). They may 

 differ from one another in various ways but all agree in containing 

 an abundance of available water. In response to a degree of uni- 

 formity of environment plant forms have developed with certain 

 characteristics that may be taken as more or less distinctive of plants 

 of hydric habitats. 



A pond or small lake may be taken as typical of extreme hydric 

 habitats. Characteristic plants of such a habitat are many algse, 

 duckweeds, pond lilies, etc. Root systems are much reduced both 

 in length and in amount of branching. Root hairs are absent in the 

 water though they may be present where the roots extend into the 

 mud below. Leaves often equal or surpass roots as absorbing organs. 

 Those of submerged plants are thin and often finely dissected. Air 

 spaces often exceed the tissue in actual volume. Stomata are absent 



(167) 



