CHAPTER LV. 



PLANT COMMUNITIES: SEASONAL CHANGES. 



719. One of the interesting subjects for observation in the 

 study of the habits and haunts of plants is the relation of plants 

 to each other in communities. In the topography of the moors, 

 and of the land near and on the margins of bodies of water, we 

 have seen how the adaptation of plants to certain moisture con- 

 ditions of the soil, and to varying depths of the water, causes 

 those of a like habit in this respect to be arranged in definite 

 zones. Often there is a predominating species in a given zone, 

 while again there may be several occupying the same zone, more 

 or less equally sharing the occupation. Many times one species 

 is the dominant form, while several others exist by sufferance. 



720. Plants of widely different groups may exist in the 

 same community. — So it is that plants of widely different rela- 

 tionships have become adapted to grow under almost identical 

 environmental conditions. The reed or grass growing in the 

 water is often accompanied by floating mats of filamentous alga; 

 like spirogyra, zygnema ; or other species, as cedogonium, coleo- 

 chaste, attach themselves to these higher lords of creation ; while 

 desmids find a lodging place on their surface or entangled in the 

 meshes of the other alga;. Chara also is often an accompaniment 

 in such plant communities, and water-loving mosses, liverworts, 

 and fern-like plants as marsilia. Thus the widest range of plant 

 life, from the simple diatom or monad to the complex flowering 

 plant, may, by normal habit or adapted form, live side by side, 

 each able to hold its place in the community. 



721. In field or forest, along glade or glen, on mountain 

 slope or in desert regions, similar relationships of plants in 



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