CLIMBING PLANTS. 



701 



the trunks of old trees, steep walls of rock, and under cultivation often wooden planks 

 and palings. All these climbing stems have two kinds of roots— absorbent roots, by 

 means of which they suck up fluid food, and climbing roots, which serve to maintain 

 them on their supports. In most instances the functions of these two kinds of roots 

 are kept distinct, so that a climbing stem soon withers and dies when it is cut across 

 above the absorbent roots, although affixed to a rock or to the bark of a tree by a 

 thousand climbing roots. But in a few cases the climbing roots will under these 

 circumstances begin to absorb, provided, of course, that the substratum to which 

 they adhere is able to afford them the necessary food. 



In many respects climbing stems agree with the group of tendril-bearing stems 

 just described, especially in the fact that the organs which furnish the adhesion to 

 the support turn from the light, and also inasmuch as the adhesion to the support 

 is brought about by a viscous substance either secreted by the cells in contact or 

 produced by the breaking down into mucilage of the outer layers of the walls of 

 these cells. The avoidance of light by all climbing roots is an extremely interesting 

 fact. Whether the stem which forms climbing roots nestles close to its substratum, 

 or some spans distant from it, whether it grows upwards along a stone wall or 

 is deflected to one side by some obstacle— in all cases the first rudiments of the 

 climbing roots make their appearance on the side of the stem turned away from 

 the light. And when these small cushions develop into root-fibres, the direction 

 assumed by their growth is always away from the liglit and towards the dark back- 

 ground. The darker the place, the more vigorous do the root-fibres become. When 

 the climbing roots developed by Tecoma radicans (figured on p. 479) in the darkest 

 places under a projecting ridge are compared with those which have been formed in 

 less shaded places below, it is seen that the former are always much more luxuriant 

 and longer than the latter. If by chance a shoot which has already begun to 

 develop climbing roots is moved from its position so that the hitherto shaded side 

 is exposed to the light, it twists round until the side with the rudiments of aerial 

 roots is again turned from the light. If obstacles lie in the way of this torsion, the 

 young climbing roots thus exposed remain undeveloped and grow no further; ulti- 

 mately they wither and dry up. 



As soon as the climbing roots originating from the shady side of the stem come 

 into contact with the substratum below them their growth is noticeably increased, 

 and in a very short time they become firmly united to it. Not only do the rootlets 

 grow into all the crevices of the substratum and adapt themselves most accurately 

 to its larger inequalities, but each single epidermal cell of the growing rootlet 

 exhibits a like behaviour, fits itself to the smallest projections and depressions, 

 and spreads out on entirely smooth surfaces like a plastic mass. Sometimes the 

 epidermal cells are drawn out like tubes and form so-called root-hairs, these penetrate 

 into the smallest clefts of the substratum and spread out like a hand whose palm 

 and outspread fingers press against the soil. These epidermal cells of climbing 

 roots also unite with the supports against which they have placed themselves like 

 the absorbent cells described on p. 87, and the union is so firm that the roots are 



