20 THE STEUGGLE FOE EXISTENCE. 



tion, i. e. the shorter the season of vegetation, the more contracted 

 will the crowns be and the further must the trees stand apart, as 

 near the Polar Regions and at the upper limit of tree vegetation. 



Lastly, the greater the intensity of sunlight at any place is, the 

 better can a given species tolerate cover overhead ; but as increasing 

 brightness of the sun's rays necessarily means a corresponding in- 

 crease of heat, which in its turn means, in most parts of the empire, 

 a dry soil and a dry atmosphere, the beneficial effect of the inten- 

 sity of light is nearly everywhere more than counteracted by the un- 

 favourable influence of drought. We hence see the apparent contra- 

 diction of a closer leaf-canopy and a denser ground-vegetation on 

 the northern slopes of hills than on their southern slopes. The denser 

 growth at high elevations, although in a very large measure due 

 also to the presence of more moisture in the soil and air during the 

 dry season of the year, is to a great extent the result of the brighter 

 light in the clear and rare atmosphere there. 



If the species composing the crop is shade-enduring, like the 

 deodar, silver fir, Quercus inoana, Terminalia tomentosa, &c., or one' 

 which at all ages interlaces branches, Hke the khair, species of Zizy- 

 phus, Anogeissus pendula, A. latifolia, &c., the struggle for existence 

 is a long and severe one. On the other hand, when the constituent 

 species is like Pinus longifolia or Hardwichia hinata, the first shade- 

 avoiding from its earliest years, and both shrinking from the interla- 

 cing of branches, the struggle is a brief one, and the weaker and over- 

 topped plants are soon thrown out of the race. And, in general, 

 the severity and duration of the struggle for existence is in direct 

 proportion to the tendency of the trees to grow close together, what- 

 ever the cause or causes that tendency may be due to. 



V. Spread of roots (including the rhizome). — Some species, 

 such as sal and Prosopis spicigera, develop a main root which works 

 its way vertically down to a considerable depth ; while others, after 

 sending down a short taproot that ultimately dies, produce only 

 lateral roots, which start from just below the base of the stem and 

 run along close and more or less parallel to the surface of the ground. 

 These latter, as distinguished from all other plants, may be called 

 surface-feeders, since they depend for their sustenance entirely on 

 the upper layers of the soil. Between these two extremes come all 

 the remaining plants, the main roots of which diverge at different 

 angles from the vertical or horizontal. It is evident that the more 

 nearly a species assumes the character of a surface-feeder, the earlier 

 will the struggle for existence begin, for in the contrary case a plant 

 can go down to lower depths for its food, if it has to yield to lateral 



