82 THE STRUGGLE FOE EXISTENCE. 



diffusion of the elaborated sap coming down from the leaves and 

 other green parts; and, finally, the channel for the passage of the 

 crude sap from the roots becomes disproportionately smaller and 

 smaller for the nourishment of the crowns, which become, in conse- 

 quence, scraggy, sickly, and full of dead twigs and branches (the 

 cause of future interior decomposition and attacks of insects and 

 fungi), and, if the trees form a heartwood, there is 'ultimately no 

 sapwood left in their trunks and death from sheer starvation is the 

 result. 



As with all other plants, so with climbers, the immediate effect 

 of light is to cause early and, as far as the species in question is 

 concerned, complete lignification, and thus to check the enlarge- 

 ment of their tissues, which are indeed in most, if not in all, cases, 

 naturally of looser and more herbaceous texture than those of erect 

 woody plants; and hence many climbers exposed to full sunlight 

 and wanting a support become low erect bushes (Millettia, Bauhi- 

 nia, Zizyphus, &c.), or at least assume only a straggling habit 

 ( Spatholobus, Butea, &c.j With defective illumination, stopping 

 short of the minimum absolutely necessary for assimilation in each 

 case, climbers will form long internodes, and extend themselves 

 vigorously; and indeed the largest of them are found in dense over- 

 green forests. Taking the special case of twiners, we know that 

 the nutating end ot the stem has comparatively small leaves (111 

 Bauhinia none at all) and long internodes, exactly as if they were 

 growing in weak light, and this character is no doubt the result of 

 a habit still continued, although the inducing cause ( viz., deficient 

 illumination) is absent. Moreover all climbers are shade-enduring, 

 for without that quality they could not, as they actually do, make 

 their way up through the very densest crowns, as well as develop 

 an aggressive mass of leaves in the midst of the already abundant 

 foliage of those crowns. 



A further characteristic of many climbers, especially those which 

 twine or scramble up, is that they grow very rapidly in length. 

 Thus, for instance, Bauhmia Vahlii often attains a length of more 

 than 50 feet in a single season, and throws up long shoots which 

 are at once able to reach the crowns of shrubs and low trees. It is 

 this property especially which enables such climbers to attack and 

 overcome trees that are even more shade-enduring than themselves 

 (see p. 47, para. 2). 



To sum up: Climbers, no matter how they ascend, are able, thanks 

 to their being shade-enduring to a very remarkable degree ami 

 often capable of rapid elongation, to rise up into, invade, and over- 



