PCEE CEOP OF UNIPCEM AGE. 19 



If 5? SO than their immediate neighbours, will succumb under cOvor_ 

 Tlif. struggle will evidently be severest on the side where the leat- 

 canopy is closest. 



Individuals of every species whatsoever, even the most shade- 

 avoiding, can grow very much closer together when young than at 

 a later age, their need for light increasing with advancing years ; for 

 the smaller the amount of nourishment to be elaborated is, the small- 

 er will be the sum total of light required, and vice versa. In a cano- 

 pied crop, as long as a tree is still growing in height, light reaching 

 it from above suffices, if not to enable it to grow luxuriantly, 

 at least to keep it alive and to draw it up rapidly. But when the 

 phase of upward growth is practically over, that is to say, when the 

 upper and sole illuminated portion of the crown has lost its excep- 

 tional vigour and can no longer either extend itself and thus replace 

 by new organs the lower branches which have hitherto died and fal- 

 len ofp without any constitutional injury to the tree, or assimilate 

 sufficient nourishment for the parts below, the tree can continue to 

 live only on the condition that the lower portions of the crown are 

 able to spread out and develop a large leaf-surface and thus contribute 

 their share towards its nutrition. Thenceforth a keen struggle for 

 lateral development ensues with its neighbours, and the tree gains 

 or loses the victory according as it is stronger or weaker than they, 

 that is to say, according as it can or cannot press in their crowns 

 and ultimately kill out one or more of them. Thus the leaf-canopy 

 becomes gradually sparer, until, at a very advanced age, varying 

 with the species concerned, the individual trees stand completely 

 isolated. 



The more favourable the soil and locality are, the closer together 

 can the trees stand at any age and the more tolerant will they be of 

 cover; for the spread of their roots will be in inverse proportion 

 to the abundance and concentration of food in the soil. And, in 

 general, the presence of sufficient moisture in the soil and in the at- 

 mosphere, combined vrith the requisite temperature, increases in a 

 very conspicuous degree the ability of a species to form a dense leaf- 

 canopy at all stages of growth, not only by encouraging the develop- 

 ment of foliage, but also by enabling the trees to absorb all the water 

 they need with a proportionately restricted root-apparatus. 



All species form a denser leaf-canopy the longer the period of 

 vegetation in each year is, for the total effect of light being propor- 

 tionate to the length of exposure of the trees to its action, the outspread 

 of the crowns will be greater and the foliage fuller; hence, converse- 

 ly, the total effect being the same, the shorter the time of its ao- 



