342 LECTURE XV. 



the concave side being shortened and the convex side length- 

 ened ; by repeated bending they may be moulded into almost 

 any form. A familiar illustration of the flexibility of growing 

 organs is afforded by the pendent position of many flower- 

 buds on their peduncles or pedicels ; the position is due to 

 the fact that the flexible stalk is curved by the weight of the 

 bud. Curvature of a growing organ can also be induced by 

 concussion. If an erect growing shoot be struck laterally, 

 a curvature, which persists for a time, is the result. Pril- 

 lieux has pointed out that if the shoot be struck near the 

 apex, its curvature is such that it is convex on the side upon 

 which the blow fell : if, however, the shoot be struck near its 

 base, the vibration travels upwards in the form of a wave, and 

 the curvature of the apical growing portion is such that its 

 concavity is on the side which was struck. 



With regard to the distribution of extensibility in growing 

 organs De Vries has shewn that, in growing stems, the maxi- 

 mum of extensibility and of flexibility exists somewhat behind 

 the punctum vegetationis, that is, in the region of most active 

 growth. As the tissues become mature their extensibility 

 diminishes and their elasticity increases. 



3. Tensions in Growing Organs. 



We have learned that a state of tension, turgidity, as it is 

 termed, is an essential condition of growth. All growing 

 organs, therefore, exhibit this form of tension. But we have 

 now -to deal with tensions of another kind which are due, 

 in the first place, to the fact that the cells do not all tend 

 in the course of their growth to attain the same form, but 

 to assume different forms. At the apex of a growing stem, 

 for instance, some of the cells increase in all dimensions to 

 form the kind of permanent tissue which is known as paren- 

 chyma, whereas others increase especially in length to form 

 the kind of permanent tissue which is known as prosenchyma. 

 Further, this histological differentiation is accompanied by 

 differences in the chemical composition and in the physical 

 properties of the cell-walls. The walls of the epidermal cells, 



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