226 ZONES AND REGIONS [Pt. Ill, Sect. 



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

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

 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 rain 

 season ; one ma}' see in this an effect associated with the increased onibrophoby 

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

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

 rainy season. It is on the whole very probable that in many cases in nature, n 

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

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

 be more closely investigated. 



3. THE FLORISTIC CHARACTER OF THE TROPICAL ZON: 



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

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

 zones enclosed by the tropics possess a pronounced megathermic flo 

 which in places, for instance in Southern Plorida and Southern Bras 

 extends somewhat beyond the tropics. Yet in these latter extensic 

 the megathermic flora is already perceptibly impoverished, because t 

 annual curve of temperature no longer corresponds to the oecologiil 

 optimum of many species. In particular, the temperature necessary 

 the ripening of fruit is frequentlj' not maintained. 



The following summar)- gives the general characteristics of the mesi 

 thermic floras, as it enumerates in systematic order the families t!t 

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

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

 because a separate chapter is devoted to them ^ 



Thallophyta. 



Though Algae as terrestrial plants are of merely subordinate importa 

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

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

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

 investigated. Yet it seems to be already ascertained that the lai-r 

 orders known to exist in Europe are all represented. In other resp'ts 

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

 Europe are very prominent, especially among the Hymenomycetes, re 

 feebly represented in the torrid zones. 



The following remarks of Alfred Moller regarding the fungal vegetajjn 

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

 forests that I know : — \ 



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

 eye of an observer who is not specially looking for them, and that e.\ercise conS|:r- 



' See Sect. V. I 



