252 PLANT RELATIONS. 



with moisture. In such conditions pure forests may oc- 

 cur, characterized by such tree forms as the tree ferns, 

 palms, or bamboos. Only the great mixed tropical forest 

 will be considered. The main characteristics are as fol- 

 lows: 



(1) Absence of simultaneous periodicity. — Perhaps the 

 most striking feature, in contrast with the deciduous for- 

 ests, is that there is no regular period for the develop- 

 ment or fall of leaves. Leaf activity is possible through- 

 out the year, and there is no time of bare forest, or of 

 forests just putting out leaves. This does not mean that 

 the leaves persist indefinitely, but that there is no regular 

 time for their fall and formation. Leaves are continually 

 being shed and formed, but the trees always appear in full 

 foliage. 



(2) Density of growth. — Such an area is remarkably 

 filled with vegetation stratum, after stratum occurring, 

 resulting in gigantic jungles. The higher strata may be 

 made up of trees of different heights, below them are shrubs 

 of varying heights, then tall and low herbs, and finally 

 mosses and liverworts. Among these close-set standing 

 forms, great vines or lianas climb and bind the standing 

 vegetation into an inextricable tangle (see Figs. 55, 212). 

 In addition to these, hosts of aerial plants find lodging 

 places upon the tree-trunks and vines (see Fig. 213). These 

 rainy forests of the tropics furnish the very best conditions 

 for the development of the numerous epiphytic orchids, 

 bromelias, etc. In such conditions also numerous sapro- 

 phytes occur. Such an assemblage of vegetation is to be 

 found nowhere else. 



(3) Number of species. — Not only is there an immense 

 number of individuals, but an extraordinary number of 

 species occur. A list of plants growing in these forests 

 would show a remarkable representation of the plant 

 kingdom. 



(4) Forms of trees. — The dense vegetation results in 



