;ui MECHANICAL LAWS OF GROWTH. 



diminishes it. If we imagine a straight succulent stem or a growing root to be bent, 

 the cells on the convex side will be stretched, those on the concave side compressed, 

 and the turgidity will be diminished in proportion in the former and increased in the 

 latter. This result is very clearly confirmed if a very succulent rapidly growing 

 internode of the grape-vine is slowly but firmly bent till it describes about a semi- 

 circle. It will be observed that during the bending a number of small drops of 

 W'ater escape in rows from the epidermis on the concave compressed and shortened 

 side. It is indifferent whether they escape through fissures or are forced out through 

 the cell- walls; in either case they show that the cells display a higher degree of 

 turgidity on the concave compressed side than when the internode was straight. 



In the present state of our knowledge, if we would keep clear of uncertain 

 speculations, the considerations now given must be considered as by no means 

 complete ; but they are sufficient to draw the attention to processes which must be 

 taken into account in the interior of the growing' parts of plants when they are 

 subject to pressure, traction, bending, and so forth, from external forces. But 

 if these internal changes are for the time left out of account, the purely, external 

 effect of the forces already mentioned is deserving of greater attention than it has 

 hitherto received^ It would be of essential service, for instance, to ascertain at 

 what point a growing internode, root, leaf, &c., possesses the greatest extensibility, 

 flexibility, and elasticity, and whether this point coincides or not with that of the 

 most vigorous growth, and how perfect is the elasticity of the part ; and so forth. 

 We shall see that even somewhat crude observations in this direction afford results 

 which enable us to remove old errors and avoid new ones. 



Compared with the extensibility of mature internodes and parts of internodes, 

 that of rapidly growing parts is very considerable, but their elasticity, on the con- 

 trary, is very imperfect. But the greater the development of the wood of a growing 

 part, the greater is its elasticity and the less its extensibility. In young non-lignified 

 roots, on the contrary, the resistance to bending is greater in the youngest than in 

 the older parts, especially those whose growth in length has long been completed. 

 The extremities of roots, very young leaves, and the ends of stems still enclosed 

 in the bud, are generally brittle under a blow or pressure, but pliable and plastic to 

 long-continued action of this kind, a condition that gives place during growth to 

 an increasing resistance to sudden blows, which is in the first place due to in- 

 crease of extensibility, afterwards to increase of elasticity. 



In rapidly growing stems, leaves, and roots, the Hmit of elasticity is easily over- 

 stepped even by momentary flexion; and they always retain afterwards a slight 

 though distinct curvature. It is often even possible, especially with roots and slender 

 internodes, to give them any desired form by repeated bending with the fingers in 

 different directions, like a thread of wax or a red-hot iron ware, without the power 

 of growth being at all injured by the process. This effect is attained with greater 

 certainty by exerting on the growing structure a flexion which is prolonged although 

 small in amount. Thus the pedicels of many flowers are bent downwards by their 

 weight, and retain this curvature even when the w^eight is removed, until a new con- 



* See A. P. De Candolle, Physiologic Vt^getale, vol. I, p. ii. 



