-28 PLANT GEOGRAPHY 



physiological requirements and response in growth or 

 energy of each species may be different and so require 

 to be differently stated, but also that the life of any 

 plant is made up of an infinite number of phases, each of 

 Avhich may have its separate requirements and limits; 

 the lowest, best, and highest degrees of intensity of light, 

 of temperature, or of moisture, for instance, at which 

 it can take place. 



A. LIGHT 



Light depends mainly upon latitude, the obliquity of 

 the sun's rays increasing and the intensity of light 

 diminishing, from the equator towards the poles; while 

 the length of the day, i.e. the duration of light, increases 

 in proportion. Thus in polar regions the sun daily 

 describes an almost complete circle in the sky, its light 

 falling upon plants from every side in turn, though at a 

 low angle. This is termed " circumpolar light." 



Although light varies less within the limits of one lati- 

 tudinal zone than will heat or moisture, it may be 

 .greatly diminished by the shade of trees or other plants, 

 whether all the year round, as in the case of the ever- 

 green forests of the tropical regions, or during the 

 summer, as in the deciduous forests of temperate 

 latitudes. Under these circumstances, also, filtered, as 

 it were, by passing through the overhanging foliage, it 

 will cease in the main to be white light or to consist of 

 rays of various refrangibilities, some rays being absorbed. 

 So, too, in passing through water of any depth some 

 light is reflected, some absorbed, and all refracted; so 

 that submerged plants are much in the position of 

 shade plants. 



While the duration of light varies with the season, its 

 intensity increases in the rarefied air of mountain heights, 

 while the presence of moisture in the air, whether 

 invisible or condensed as cloud, also exerts a modifying 

 effect upon light. 



Not even in deep sea is it apparently too dark for 

 bacterial life, nor in caves for the development of 

 moulds ; but green plants, requiring light for the forma- 

 tion and action of their chlorophyll, are absent from 

 such situations. When light is small in amount plants 

 may apparently develop structures specially adapted 



