i 



GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 783 



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

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

 itherto 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 wire, 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 weight is removed, until a new con- 

 dition of growth imparts greater elasticity and firmness to the tissues : under the 

 influence of gravitation they then grow more rapidly on the lower side, become 

 upright, and raise up the still greater weight of the fruit; as is strikingly seen 

 in Fritillaria wipenalis, Anemone praiensis, and many other plants with pendent 

 flowers and erect fruits. In other cases again the curvature, which was at first 

 due merely to external causes, becomes permanent and fixed in the tissue itself 

 by the processes of growth, as in the fruit-stalks of Solanum Dulcamara. 



One of the most striking phenomena of this class is that a lateral blow below a 

 growing internode causes it to assume a curvature in the direction assumed by the 

 internode at the moment of impact. The same thing occurs when the upper part of 

 a shoot is taken in the hand and a curvature imparted to it similar to that caused by 

 the blow. The upper part acquires in consequence a pendent position, which may 

 however be again neutralised by subsequent growth. 



There has been as yet no exact or detailed investigation of the elasticity of 

 growing shoots, roots, and leaves; and the enquiry is, as I have convinced myself, 

 attended with considerable difficulty. Observations sufficient to enable us to study 



See A. P. De Candolle, Physiologie Vegetale, vol. I. p. 11. 



