Chap. I] CHARACTERS OF THE TEMPERATE CLIMATE 419 



increased production of pigments in flowers and fruits, as well as of 

 ethereal oils, near the north polar circle, is, probably rightly, attributed 

 to the longer duration of light 1 . 



iii. EFFECTS OF ATMOSPHERIC PRECIPITATIONS. 



Atmospheric precipitations determine, in the first place, the distribution 

 of woodland, grassland, and desert, also the vegetative character of their 

 individual formations, within the temperate zones ; their significance is 

 however somewhat less than in the tropics, for this is evidently dependent 

 on the temperature prevailing at the time of the precipitations, so that, 

 in both temperate zones, districts with summer rain and dry winters 

 contrast most sharply in their vegetation with districts having winter rain 

 and dry summers -. 



An amount of precipitation that in the tropics would occasion a most 

 luxuriant development of vegetation, has no such invigorating effects on 

 plant- life in the temperate zones. This difference depends in particular 

 on the fact that the cold of winter corresponds physiologically to a pro- 

 nounced dry season, and accordingly sets a decided limit to the surface 

 growth of plant-members. 



The periodic phenomena of plant-life, which in the tropics are regulated 

 exclusively by the alternations of moist and dry seasons, are also partially 

 dependent on these in temperate districts with mild winters, although 

 even in such districts change of temperature makes itself felt. Temperature 

 has the greater effect, or is alone effective, in districts with cold winters, 

 according as they possess a dry or a wet summer. 



In a similar way, the importance of humidity recedes before that of 

 temperature in the demarcation of the areas of mesothermic species of 

 plants. Only in climates with markedly mild winters are there found 

 groups of plants whose distribution is solely determined by atmospheric 

 precipitations, as is the case in the tropics. 



3. CHARACTER OF THE FLORA OF TEMPERATE ZONES. 



I will now give a condensed summary of the mesothermic groups of 

 forms, which are treated in a manner like that adopted when dealing with 

 the tropical zones, and from the standpoints already given on p. 226. 



Thallophyta. 



Algae are even less developed in temperate than in tropical terrestrial 

 floras, except when they combine with fungi to form Lidiencs, which rapidly 

 increase in the number of their species and individuals, as the climate 



1 Schuebeler, op. cit., p. S3. 2 See Part III, Sect. II, Chaps. III-Y. 



E e 2 



