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



vital processes as a whole, but only resting periods for certain functions. 

 A plant during its hibernation is by no means inactive. In many trees, 

 starch is converted into oil ; the chlorophyll-corpuscles of conifers produce 

 red colouring matter ; the epidermis of the leaves of many herbs produces 

 cyanophyll ; the roots continue to grow in length ; the winter-buds, by 

 means of invisible processes, acquire the power of further development, 

 which was lacking in them in the warm season, and so on. On the other 

 hand, repose chiefly prevails as regards the inception, the growth in length 

 and thickness of the shoots, although there are exceptions. Assimilation 

 is weakened, yet suspended only in hard frosty weather. 



At spring-time, in temperate latitudes, the period of rest for the processes 

 of growth terminates ; leafy shoots and flowers are formed ; the functions 

 of nutrition, especially the conversion of already assimilated material, break 

 into stirring activity. On the other hand, functions depending on lower 

 temperatures enter on a period of rest. 



During summer, the greatest activity prevails in the domain of nutrition, 

 especially as regards the assimilation of raw materials. Growth in thick- 

 ness of stems, and in thickness and length of the roots, is in full activity ; 

 on the contrary, after the winter-buds have been completed, the meristems 

 of the terminal buds are, as a rule, at rest. As regards the domain of 

 reproduction, there may be activity or repose, according to species. 



Autumn is a period of slackness, of the commencement of repose in most 

 domains. In some species, however, reproductive activity for the first time 

 awakens from rest. 



There is therefore in the temperate zones no season of the year that docs 

 not tend to set in motion certain functions of plant-life, and to set others at 

 rest. During autumn and winter repose predominates, and during spring 

 and summer activity predominates, so that we may speak of relative seasons 

 of rest and of vegetation, although at no season is either condition actually 

 realized. 



Tropical plants arc just as subject to the periodic alternation of rest and 

 of activity as arc those of the cooler and cold zones. Wherever a sharp 

 climatic periodicity prevails the functions of the plant-organism in the 

 tropics also appear to be decidedly influenced by it. Thus dry seasons act 

 like cold ones in many respects. The less marked the periodicity of the 

 climate is, the less dependent on its influence is the periodicity in the plant. 

 Internal causes are mainly or solely responsible for the alternation of rest and 

 of activity in a nearly uniform climate. Such a rhythmic change is how- 

 ever never abandoned, for it arises from the nature of the living organism 

 and not from external conditions ; its connexion with external conditions 

 is a secondary feature — an adaptation. 



Hence the picture of general and continuous activity which most 

 travellers have brought with them regarding the vegetation in constantly 



