2 20 



LECTURE XIII. 



of construction of shoot-axes and leaves here referred to, showed how the distri- 

 bution of the elastic strands and layers strictly accords with the principles of 

 theoretical mechanics. That the mechanical elements effective in the construction 

 of thin stems also find their use in the construction of leaves, is easily intelligible ; 

 in this, however, it is a matter not merely of the elasticity and rigidity which 

 determine their free suspension in the air, but also of that kind of solidity which 

 prevents the tearing from the margin inwards. It has already been explained in a 

 previous lecture (IV), with a series of examples, how foliage leaves are protected 

 against such injury by the mode of distribution of the ribs and veins. 



After what has been said, it scarcely requires special mention that the kind of 

 rigidity last described, since it depends on the subsequent lignification of the elastic 

 fibres, only comes into play in the older stems and in leaves which are already fully 

 grown ; whereas the kind of rigidity previously described, and effected by the turges- 

 cence and tensions of the tissue, serves the organs which are still elongating, or which 

 at any rate are not provided with lignified parts. For the sake of completeness I may 



add at the same time that young shoot- 

 axes and leaf-stalks in an earlier stage of 

 their growth in length, exhibit yet a third 

 kind of rigidity : this depends also, it is 

 true, on turgescence, but is characteristic 

 in that the walls of the turgescent cells are 

 more extensible and less elastic than in 

 later stages of growth. The portions of the 

 shoot-axes, some 10-20 cm. long, under 

 the terminal buds of Clematis and Aristo- 

 lochia, and under the young flowering spike 

 o{ Plantago major, &c., are in this condition 

 highly extensible. A longitudinal pull acts 

 on these organs as on a thin thread of caout- 

 chouc, and they are so flexible, like such 

 a thread, that they can literally be wound round the finger. Stretching, and strong 

 bending at the same time, however, make them limp, and effect a diminution of 

 the rigidity: this is brought about by the fact that the limits of elasticity of the 

 still very thin cell walls are easily exceeded by the stretching. On this peculiarity, by 

 which the young filiform structures obtain the consistence of a leaden wire, or perhaps 

 of a wax thread, depends an easily observed phenomenon, which Hofmeister errone- 

 ously explained as a phenomenon of sensitiveness \ If shoots with long, slender, still 

 growing apical portions are shaken, it is remarked that after the shaking they hang 

 down limp, to become upright again, in consequence of further growth, only after some 

 time. I showed several years ago that the apparently irritable curvature of young shoot- 

 axes is nothing more than an effect of their great extensibility, in combination with 

 very small elasticity, and that one cannot here speak of sensitive effects. That such 



Fig. 191. — Transverse section of a petiole of Mitsa Eiisete. 

 The lamella: of tissue at the periphery together form an elastic 

 system ; between these are stretched soft parenchymatous 

 plates, in which run thin vascular bundles. 



' The bending down of the growing apices of shoots, caused by shakings or blows, was first 

 closely investigated by Hofmeister, but erroneously regarded as a phenomenon of irritability. 

 I gave the true explanation of this phenomenon from the great flexibility and feeble elasticity of 

 growing shoot-axes in the third edition of my 'Text-book,' 1873. 



