PURE CROP OF MIXED AOKS. 29 



they may gain now vigour by being cut back, otherwise, although 

 they nitiy continue to live, they will never, under the most favour- 

 able circumstances afterwards, attain any development. 



The injuriousness of cover is directly proportional to the combined 

 lowness and depth of the crowns of the trees constituting it, sin o 

 more abundant light can filter through a high and thin leaf-canopy, 

 under which moreover there can never be any deep shadows. 

 Hence vigorous overtopped individuals may have a very good 

 chance of surviving, and perhaps even of ultimately catching up, 

 their overtopping neighbours, if these have high narrow crowns. 



Due weight being given to the important considerations developed 

 in the preceding paragraphs, all the remarks made under this head 

 in the First Case are to be accepted as holding good here also, and as 

 even applying with greater force. Only a single additional obser- 

 vation need be made in this place as a pendant to the proposition 

 that young plants of every species whatsoever are more shade- 

 enduring than their older fellows. It is that this difference is 

 more marked in proportion to the partiality of the species in ques- 

 tion for light. 



V. SPREAD or ROOTS (INCLUDING THE RHIZOME). This is a 

 much more important condition than is very generally supposed, 

 its most striking effects being erroneously attributed to other causes, 

 notably to sufficiency or insufficiency of light. On the ground 

 immediately surrounding a large sal tree, although many sal seeds 

 germinate every year, hardly any of the numerous plants produced 

 are observed to survive. This result is at once ascribed to deficient 

 illumination due to the cover of the large tree, notwithstanding that 



O ' O 



the tree in question may be quite isolated, unbranched up to more 

 than 40 feet, and narrow-crowned. The true reason is to be sought 

 for in the dense and thick mass of roots, root-fibres and root-hairs of 

 the large tree, which form a close network occupying the soil to a 

 very considerable depth, defying penetration by the thin weak roots 

 of the seedlings, and monopolising, thanks to its very much greater 

 absorptive power, all the nourishment that the soil can yield.* The 

 same phenomenon may be observed with all species, more markedly 

 with some than with others. We can now understand why the in- 



* If further proof were required to show the utter groundlessness of the reason 

 generally ascribed, we have Only to look at the frequently flourishing young sal under 

 the really dense cover of trees of other sptcies possessing very much heavier crowns 

 than the sal, but which, having a different root-system and different requirements, 

 do u >t prevent the roots of the y juug sal from getting through into soil not completely 

 monopolised by other plauts. 



