226 ZONES AND REGIONS [Pt. Ill, Sect. I 



become wettable and ombrophilous, but in advanced age reassume the characters of 

 their youth. Hence after heavy rainfall they readily die and fall to the ground, since, 

 in contrast to the young leaves, they are not protected against the rain by their lie. 



Many trees in periodically dry districts shed their leaves even during the rainy 

 season ; one may see in this an effect associated with the increased ombrophoby of 

 age. On the other hand, I have met with the statement here and there in books 

 of travel that certain trees or even entire forests become leafless at the height of the 

 rainy season. It is on the whole very probable that in many cases in nature, not 

 drought but great humidity may be the cause of the periodic leaf-fall. It is desirable 

 that in continuation of Wiesner's researches the cause of leaf-fall in the tropics may 

 be more closely investigated. 



3. THE FLORISTIC CHARACTER OF THE TROPICAL ZONE. 



With the exception of some border districts, usually of limited area, 

 where the winter temperature regularly descends to freezing-point, the 

 zones enclosed by the tropics possess a pronounced megathermic flora, 

 which in places, for instance in Southern Florida and Southern Brazil, 

 extends somewhat beyond the tropics. Yet in these latter extensions 

 the megathermic flora is already perceptibly impoverished, because the 

 annual curve of temperature no longer corresponds to the oecological 

 optimum of many species. In particular, the temperature necessary for 

 the ripening of fruit is frequently not maintained. 



The following summary gives the general characteristics of the mega- 

 thermic floras, as it enumerates in systematic order the families that 

 occur in tropical lowlands, and briefly sketches the part they play as 

 regards number of species and of individuals. Aquatic plants are omitted, 

 because a separate chapter is devoted to them 1 . 



Thallophyta. 



Though Algae as terrestrial plants are of merely subordinate importance 

 in the tropics, yet their significance, at least in rainy districts, is always 

 greater than in other latitudes. Many species live as epiphytes, chiefly 

 on leaves. The Fungi of the tropics have not yet been satisfactorily 

 investigated. Yet it seems to be already ascertained that the larger 

 orders known to exist in Europe are all represented. In other respects 

 deep-seated differences are not wanting, and many groups that in Central 

 Europe are very prominent, especially among the Hymenomycetes, are 

 feebly represented in the torrid zones. 



The following remarks of Alfred Moller regarding the fungal vegetation 

 of the forests near the coast of South Brazil hold good for all tropical 

 forests that I know : — 



' On wet autumn days we find in our German forests far more fungi that strike the 

 eye of an observer who is not specially looking for them, and that exercise consider- 



1 See Sect. V. 



