TENSIONS OF TlSSr/ES. 21 7 



however, are limp as damp paper : the uncovered inner succulent body of tissue 

 is now likewise highly flexible; it is quite impossible, for example, to hold it 

 suspended horizontally, since it at once bends limp downwards. We have here 

 then the case of an elastic, solid, stiff body consisting of two parts, each in a high 

 degree flexible and by no means stiff: only in their natural connection do the 

 epidermal and internal tissues together form an elastic and rigid body ; and in fact it 

 is the mutual tension, and the circumstance that the inner tissue is strictly too large 

 for the extensible epidermis (or, conversely, the epidermis too small for the former), 

 which brings about the rigidity of the whole. The same is also the case, however, 

 with a turgescent cell. Its membrane taken by itself is flaccid, and of course we 

 cannot speak of solidity in connection with the fluid contents ; nevertheless a tur- 

 gescent cell is as elastic as a billiard ball. We have the same condition of affairs also 

 in a thin-walled caoutchouc balloon, which, when empty, is a limp wrinkled sac, but 

 which may be converted into a firm elastic sphere by being strongly inflated with air : 

 the solidity of this again depends simply upon the mutual pressure of contents and 

 skin. If we suppose some hundreds of thousands of small caoutchouc balloons 

 thus inflated with air, and all contained together in an extensible caoutchouc vesicle, 

 the latter, together with its contents, would also form a rigid bar like the stem of a 

 plant. If we suppose the small caoutchouc balloons not inflated with air, but filled 

 tense with water, the same effect results ; and it is somewhat in this manner that we 

 have to imagine the rigidity of a petiole or stem produced by the turgescence of 

 the cells. It is at once clear that if the small balloons in the supposed system lose 

 a part of their turgescence by the withdrawal of water, and each thence becomes 

 somewhat smaller, that the tension of these cellular contents towards the enveloping 

 caoutchouc vesicle also diminishes ; the latter would then become shorter, and the 

 rigid system must at the same time relax. We must picture somewhat in this way 

 the drooping of a cut-off shoot when it loses water by evaporation. 



The rigidity of succulent shoot-axes and leaves, especially those which are 

 still growing in length, depends essentially upon this condition of the layers 

 of tissue, brought about by turgescence and tissue-tensions; the same is true 

 of the peculiar firmness of succulent fruits, tubers, bulbs, and roots, which all 

 become limp and soft, or as one generally expresses it, shrivel, by the loss of water. 

 The upright position of the young flowering stalks which numerous plants put forth 

 in the spring, and the rigidity of young shoot-axes and the leaves of trees in the 

 spring, are due simply and entirely to the state of things described. 



If we further suppose that in a stem, petiole or root so conditioned, a loss 

 of water takes place by evaporation from the turgescent cells only on one side of 

 the longitudinal axis, the object must become shortened a little on this side, and the 

 necessary consequence is that it becomes bent or curved, since the shortened side 

 becomes concave. In like manner the increase of turgescence and extension of the 

 cells on one side of the longitudinal axis would cause this side to become convex. 

 The latter fact may be elegantly demonstrated on the roots, 10-15 cm. long, of the 

 seedHng of the Bean, INIaize, Gourd, &c. If these are allowed to dry up for a few 

 minutes in the air, they become shortened a little through slight drooping, and if the 

 roots are then dexterously laid on the surface of water, so that only the under side 

 of the root is moistened, the cells of this side at once absorb water, and become 



