RELATION OF PLANTS TO ENVIRONMENT 



455 



and other fungi disintegrate dead plants to make humus and 

 finally plant food ; certain bacteria prepare nitrates for the higher 

 plants (see paragraph 200) . 



II. VEGETATION TYPES AND STRUCTURES. 



620. Responsive type of vegetation. In studying plants in 

 relation to environment we treat rather of the form of the plants, 

 which fits them to exist under the local conditions, than of the 

 classification of plants according to natural relationships. Plants 

 may have the same vegetation * type, grow side by side, and still 

 belong to very differ- 

 ent floristic f types. 

 For example, the cac- 

 tus, yucca, three- 

 leaved sumac, the 

 sage-brush, etc., have 

 all the same general 

 vegetation type and 

 thrive in desert re- 

 gions. The red oaks, 

 the elms, many gol- 

 denrods, trillium, etc., 

 have the same general 

 vegetation type, but 

 represent very differ- 

 ent floristic types 

 since they belong to different families and orders. The latter 

 plants grow in regions with abundant rainfall throughout the 

 year, where the growing season is not very short and tempera- 

 ture conditions are moderate. Some goldenrods grow in very 

 sandy soil which dries out quickly. These have fleshy or succu- 

 lent leaves for storing water, and while they are of the same 

 floristic type as goldenrods growing in other places, the vegetation 



* i.e., the stems, leaves, etc., are of a kind suitable for existence under the 

 same conditions. 



f Flower-structure type. 



Fig. 413- 

 Bank of ferns. (After MacMUlan.) 



