62 TffE STRUGGLE FOE EXISTENCE. 



objects altogether to any very appreciable quantity of moisture in 

 the soil, and will on that account, where the rainfall is at all plenti- 

 ful, grow only on scarps. And so on. 



But with species growing on hill sides it is not simply a question 

 of drainage. They must also possess roots adapted to hold on firmly 

 to steep slopes ; and hence of two trees, similar in every other res- 

 pect, that one will possess a decided advantage which has the strong- 

 er and more firmly-seated roots. Species with horizontally spread- 

 ing main roots will usually prosper better on slopes than on level 

 ground, since there half their large roots will be able to penetrate 

 into and hook themselves on to the Mil side, whereas on level ground 

 every such root will be superficial, e.g., oaks, Cupressus torulosa, 

 8fc. " 



On very steep slopes liable to erosion and landslips, species with 

 a close and strong network of roots, being thereby able to hold to- 

 gether the soil and rock,, can not only anchor themselves more 

 firmly than others less well endowed, but also nourish themselves 

 better. 



Gradient exercises also three other very important effects on the 

 growth of the individual plants of a crop. In the first place, on it 

 depends the largeness of the angle of incidence of the solar rays 

 and hence their heating and illuminating power, which is obviously 

 more effective the more vertically they strike the plane of the 

 ground. In the next place, gradient regulates the intensity of the 

 light reaching overtopped plants through openings in the leaf-cano- 

 py above, since, the height of the trees forming the canopy being 

 constant, the steeper the slope is, the nearer will the open sky 

 necessarily be, and vice versa, the distance in any case being the 

 product of the difference between the heights of the overtopped 

 plants and of the canopy-forming trees multiplied by the cosine of 

 the angle of slope. And in the third and last place, since the ac- 

 tual superficies of a slope is greater than the area of its horizontal 

 projection, being that area multiplied by the secant of the angle of 

 slope, the steeper the slope is, the depth, moisture, &c. of the soil 

 still continuing the same, the larger will be the number of crowns 

 capable of crowding together on the slope; for every tree, not 

 having another of gi'eater height immediately below it, will have its 

 crown free on that side, although it may be overtopped by the tree 

 immediately higher up. 



Li:-tlv, gradient affects the distribution of the seeds of the various 

 species. On a steep slope heavy and round seed will roll away, few 

 remaining in situ to reproduce the species. It is for this reason 



