22 



THE FACTORS [Part I 



The change from tints of deep green to greyish brown, from dense 

 masses of fresh foliage to dry loose branches, which the passage from a 

 hygrophilous to a xerophilous mode of life calls forth, gives a very similar 

 appearance to all tropophytic districts, in spite frequently of wide physical 

 differences in climate. Tropical luxuriance of growth is by no means 

 universal in the tropics ; extensive regions in the interior of continents 

 remind one by the physiognomy of their vegetation, even in the rainy 

 season, more of Central Europe, than of the overpowering richness of 

 regions near the coast with a heavy rainfall, and the dry season especially 

 is not at all unlike a German winter in its effects upon the covering of 

 vegetation of the soil. On the other hand, many extra-tropical tracts with 

 mild winters and abundant rainfall, such as the West of New Zealand, or 

 South Chili, possess luxuriant evergreen forests like those of the tropics. 

 There the climate is one for hygrophytes. 



Periodical foliation and defoliation is indeed particularly characteristic 

 of tropophytic districts, for the defoliation is very complete and foliation 

 very luxuriant ; but these phenomena are not confined to such districts. 

 Many xerophytic districts also possess distinct seasons, which are likewise 

 accompanied by the shedding and renewal of the foliage ; the periodic 

 change is then however less obvious, partly because the number of evergreen 

 woody plants is greater, partly because the density of the foliage is less. 

 This phenomenon is also common to many hygrophytic districts, but then 

 it is in most cases confined to a thinning of the foliage which is not com- 

 pletely shed. Besides, the phenomenon is limited to a minority of the 

 trees, except in districts which, like East Java, are climatically allied to 

 tropophytic districts and form' a transition stage to them. Defoliation 

 is an adaptation to a physiologically dry period. An attempt has been 

 made to assign as the invariable cause of its commencement the beginning 

 of an insufficient water-supply, whether due to the drying up or cooling 

 down of the soil. However obvious, a priori, this explanation may appear, 

 it is as yet an unproven hypothesis. 



2. VEGETATIVE ORGANS OF AQUATIC PLANTS i. 



The oecological conditions of plant-life in the liquid medium of water are 

 evidently different from those of plants growing in the air, however rich in 

 water-vapour this may be. Aquatic plants in fact exhibit a series of peculiar 

 characteristics that are to be correlated with the physical properties of water 

 in the liquid state. On the other hand, many characteristics that occur in 

 plants growing in a very moist atmosphere are more markedly exhibited in 

 aquatic plants also. The chemical identity of water in the liquid and the 

 gaseous condition has some influence in causing such conformity. 

 ' H. Schenck, I-III ; Goebel, op. cit. 



