70 THE STETJGGLE FOE EXISTENCE. 



matters soluble. But the most striking instance of superior ger- 

 mmative facility giving an infinitely great advantage to species 

 that possess it, is that of the grasses: whence the extreme difficulty 

 of restocking open treeless land. 



(g) Relative age of fertility. The earlier a species comes into 

 bearing, the greater the chance it has of not only perpetuating it- 

 self in the crop, but also of increasing its own proportionate dis- 

 tribution in the next generation. With the sum total of other 

 conditions against it, it can still at least successfully resist complete 

 banishment. Most shrubs and small trees enjoy this advantage, 

 without which the majority of them must inevitably be driven out 

 in time. 



Considering any one species by itself, we may say that the less 

 vigorous individuals are generally the earlier fruitful, so that those 

 growing further away from the heart of their habitat or under less 

 favourable conditions of soil or climate frequently come into bear- 

 ing earlier, a dispensation of nature that enables many species to 

 enjoy a wider distribution than they would otherwise possess. It 

 will be observed that amongst introduced species, annuals and 

 biennials generally manifest the strongest tendency to become sub- 

 spontaneous. 



XIII. RELATIVE POWER OF HEALING OVER WOUNDS. Wounds, 

 may, even if exposing living tissue, not penetrate as far as the 

 wood, or they may go as far as the wood without removing its 

 cambium layer, or they may remove the entire cambium layer. 



In the first alternative, a protective layer of cork is formed im- 

 mediately under the exposed surface, thanks to the still existing, 

 though limited, power of cell-division possessed by the green 

 parenchyna of the bark ; and this may happen at once, quite irres- 

 pective of the season of the year, provided of course the prevail- 

 ing temperature is neither excessively hot nor excessively cold. It 

 is obvious that in proportion to the thickness and vigour of this 

 parenchyma will be the rapidity with which the protective cover- 

 ing will form. As instances of rapid formation we may cite sal, 

 Terminalia f<>m:>>dosa, Boswellia serrata, Pinus long/folia, $c. 



In the second alternative, the exposed wood-cambium may be 

 covered up by new protective tissue before it can dry up, or it 

 may dry up before this tissue has had time to form. In the latter 

 event the result is the same as in the third alternative to be treat- 

 ed lower down. But when the cambium continues moist long 

 enough, the parallelopiped-shaped cells composing it are in a few 

 days transformed by active division into cubical cells forming a 



