14 



LECTURE XL. 



Stalk-like, losing their plagiotropism and curving towards the light in a way which 

 shows that they are positively heliotropic. 



In all the cases so far considered the point has always been simply that the 

 organs, under the influence of gravitation and light, make curvatures in a plane which 

 includes the direction of gravity and of the light. Very commonly, however, 

 twistings — torsions of the shoot-axes — come into existence, and these have the 

 effect of £0 turning or twisting parts of the bud which were originally arranged in 

 a vertical plane, that they finally lie in a horizontal or oblique plane. This occurs 

 quite commonly in the case of woody plants with erect stems from which horizontal 

 or oblique branches arise. The buds of these branches produce their very often 

 biserially arranged leaves in a vertical or at any rate approximately vertical plane, as 

 in Fig. 403. Now if, on the unfolding of such 

 buds, all the parts maintained their mutual posi- 

 tions, the leaves of the shoots of the second order 

 would be situated above and below on the de- 

 veloped axis ; the ordinary case, however, as found 

 in the Limes, Elms, Celtidae and very many other 

 trees and shrubs, is that the lateral shoots arising 

 from such buds extend their two series of leaves 

 in a nearly horizontal plane, which, with reference 

 to the young states (shown in Fig. 403) is only 

 possible by a twisting of the young shoot-axis dur- 

 ing elongation. Not rarely indeed it happens that 

 shoot-axes with crossed (decussate) pairs of leaves 

 undergo torsions at their internodes alternately 

 to the right and left, whence the leaves on the de- 

 veloped shoot appear to be arranged in two rows 

 only, instead of in four, and become extended in 

 a horizontal or oblique plane. 



However inadequate this treatment of the 

 anisotropy of the organs of plants must appear, 

 owing to limited space, it will nevertheless serve to 

 show how extraordinarily various are the pheno- 

 mena which are induced by the action of gravitation and light ; and I may again 

 insist that for the illustration of general ideas, I have selected a few isolated 

 examples only. These phenomena are universal in the vegetable kingdom. For 

 instance, what has been said above as to the influence of light on swarm-spores 

 may be compared with the phenomena occurring in Tropcrolum and Cucurbita, and 

 in spite of the enormous diff"erence in organisation a great similarity in their sensi- 

 tiveness to light is to be found : we saw how most swarm-spores swim towards the 

 shaded side only when the light is intense, whereas when the light is feeble they 

 turn towards the source of light, and here we find that for example the shoots 

 of Tropccohim turn away from a strong light, while they turn their apices towards 

 the source of light when it is feeble. 



I here add, in conclusion, a few words on so-called Hydrotropism, simply 

 because the experimental material to hand is not suflicient to devote a special lecture 



Fig. 403.— \'ertical section of a lateral 

 bud of a horizontal branch of Cercis cana- 

 densis in December. i,-2...-j the conse- 

 cutive leaves with their (similarly numbered) 

 stipules. The outer bud scales are omitted ; 

 the inner ones are numbered 3 3. In the 

 centre is the growing-point of the shoot, b 

 position of the leaf from the axil of which the 

 bud arises ; a a.\is of parent-shoot ; zi direc- 

 tion of gravity. 



