GEOGRAPHICAL BOTANY AS THE RESULT OF ADAPTATION. 411 



the structure in question independently by a direct response to the 

 influences of the environment. 



Taking the Tropics as a commencement, the most important features 

 of the environment are usually temperature with humidity. These are 

 uniformly of a high degree. Moreover, the economic * optimum for any 

 plant — that is, the best degree of temperature, &c, for each of its 

 functions respectively — is much the same, so that a uniform luxuriance is 

 the general result. 



In Temperate and Arctic climes the temperature may be at one time 

 high and at another very low, as in the interior of continents. The 

 plants respond to such differences, and produce deciduous trees with 

 hygrophilous t foliage in summer and xerophilous i features in winter. 

 As a rule, while a relatively high temperature favours vegetative growth, 

 lower temperatures favour reproduction ; hence temperate plants rarely 

 blossom in the Tropics. On the other hand, the flowers of high Alpine 

 and Arctic regions are notorious for their brilliant colouring and 

 abundance. 



Tropical Rmn Forest. 



The first results to be considered of the effects of a perpetually great 

 amount of heat and moisture are the " Tropical Rain Forests." In the 

 Tropics there is no " turf "as in cool temperate regions, because intense 

 heat and moisture cause the grasses to grow to a great size. This is well 

 seen in the gigantic bamboos which mount to 60 feet or more in the 

 season, growing at the rate, as measured, of 23 feet in 31 days, or nearly 

 an inch in 21 hours. In order to secure the stability of the comparatively 

 slender and hollow stems, they consist of strong cylinders with thick 

 diaphragms at intervals which prevent their collapsing, possessing a 

 minimum of material with a maximum of strength. Another type of 

 stem is that of Tree Ferns and Palms and many Dicotyledonous trees. 

 These types show the result of the intense struggle for life in the dense 

 Tropical forests in order to reach the light above. They grow tall, and 

 are branchless till they attain a great height. This form, having been 

 acquired under the conditions of the environment, is now a psrmanent 

 feature in Palms. 



As a contrast, the Doum Palm of Egypt, and Dracaenas, &c, live in 

 the open and are branched. 



Dicotyledonous trees follow the forms of Palms in having their stems 

 mostly unbranched for a great height. In some, adventitious roots 

 develop flat, vertical buttresses at the base, to resist any strain from above 

 during the severe storms. Unlike our cooler temperate forests of Pines, 

 &c, there is a dense undergrowth below, due to the intense heat and 

 moisture, notwithstanding there being a great reduction of light. There 

 are several other special features of rain forests showing interesting 

 adaptations, of which the following may be mentioned : — 



Epiphytes. — These plants, being adherent to the stems and boughs of 

 trees, partake more of a " xerophytic " character. Such, e.g., is well seen 

 in the minute foliage of epiphytal Lycopodiacecs, this being a familiar 



* I.e. best for functions. f I.e. "water-loving." 



X I.e. "drought-loving." 



e2 



