C8 THE STRUGGLE FOE EXISTENCE. 



the abundance and frequency with which they flower and seed (see 

 Condition XII, a and b). 



(c) Light, its intensity and duration. Here also it is the action 

 of extremes that exercises any appreciable influence on the struggle 

 for existence, excessively bright light having the effect of over- 

 transpiration and insufficient brightness the result of enfeebled as- 

 similation, the extreme limits being different for different species 

 and the difference being generally very much more marked in the 

 case of the lower limit. Some species cannot make any Adgorous 

 growth except under exposure to direct sunlight. These form the 

 majority of our Indian species, and include teak, sal, &c.; but 

 amongst them there are many which are able to persist, and even 

 push on gradually, with little illumination. Thus Ilardwickia bina- 

 ta, Anogeissus, &c., can resist long-continued suppression, and sal, 

 most of our climbers, and others can, unless the cover be very thick, 

 also slowly strengthen themselves and work their way up. Other 

 gpecies, however, like the Quercus incana, silver fir, many laurels, 

 &c., will flourish and attain their finest dimensions even in more or 

 less diffused light. The figs require a great deal of direct sunshine, 

 and hence their arboreal habit. Going out of India to Northern 

 Europe we have the Scots' pine and birch delighting in almost per- 

 petual sunshine during half the year, and able to survive almost end- 

 less night during the other half. As rather more than one-half of 

 India is situated within the Tropics, and of the rest the greater por- 

 tion consists of level ground fully exposed to the sun and within a 

 few degrees of the Northern Tropic, the climatic influence of light 

 on the struggle for existence, except so much of it as was consider- 

 ed under Condition IV, is of comparatively secondary importance, 

 save for those few species, such as the Pinus excelsa, deodar, and 

 some other Himalayan trees, which require bright illumination, but 

 cannot stand heat, both of which conditions are realised at the al- 

 titudes at which they grow. But this is anticipating subhead (e) 

 below. The discussion under Condition IV should be carefully 

 read in connection with the preceding observations. 



Besides affecting the relative development of the individuals of a 

 forest crop, according to the species to which they belong, the great- 

 er or less intensity of the light acting on their crowns also exercis- 

 es a powerful influence on the frequency and abundance with which 

 they flower and seed (see Condition XII, a and I). 



(d) Aspect. Change of aspect may introduce changes in all or 

 any one or more of the following conditions affecting the growth of 

 plants: diurnal and seasonal oscillations of temperature, quantity 



