GENERAL PROPERTIES OF GROWING PARTS OF PLANTS. 
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 limit 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 imperialis, Anemone pratensis, 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. ii. 
