678 JOUENAL OF THE KOYAL HORTICULTURAL SOCIETY. 



cell-wall instantly contracts and expels it. The protoplasm — which is 

 highly sensitive to a mechanical influence — is the active seat, but the- 

 cell-wall is the mechanical agent which does the work, by means of its- 

 elasticity and the tension to which it has been subjected. 



Mechan ical Structure of Leaves. — We must now consider the structure 

 of leaves, to see how they are enabled to stand out horizontally without 

 falling under the influence of gravity. "We have seen how stems respond 

 to strains ; petioles and blades will also do so. If a weight be attached 

 to a grooving leaf-stalk, it will be subsequently found to develop more 

 supportive or strengthening tissue than it would have done normally. 



Of course the petioles must be so constructed as to sustain whatever 

 weight the blade itself may have. In the gigantic leaves of many 

 palms their weight must be enormous. To do this the petiole completely 



sheathes the stem, the sheath being provided with layers of woody fibres 

 crossing each other. The whole thus gives great "purchase" to the 

 stalk. 



One of the commonest methods of strengthening the petiole itself is 

 also on the principle of a girder. It will be noticed on numerous 

 herbaceous plants, as of the Umbellifers, that the petiole is more or less 

 deeply grooved on the upper side ; this means that the petiole has " run 

 up" .apairof flanges. These act as a strengthening against any trans- 

 verse rupture by weight. 



As for the blade, it is easy to see how the "ribs " and "veins," as the 

 framewwk or "skeleton" is called, are distributed so as to support the 

 intermediate tissue and display it to incident light in the best manner 

 XDOssible. 



. Mechanical Forces displayed by Floiuers. — In studying the structure 



Fig. 290.— Calyx of Dead-nettle. 



Fig. 291. — Genista tinctoria: a, flower 

 ready for insects ; b, after ex- 

 plosion, seen from the front; c, 

 claws of keel petals, showing curva- 

 ture and contraction, causing the 

 fall of these petals. 



