■JIQ MZCHANICAL LAWS OF GROWTH. 



As we have already seen from the measurements of the layers when entirely 

 isolated, it was also evident from the curvature of the two halves of the longitudinal 

 slice that the contraction of the epidermis is less than the elongation of the pith. 

 Since this slice is somewhat longer than the entire internode, the proportionate con- 

 traction of the outside would be greater, the lengthening of the inside less. 



A rapid rate of growth, united with a certain amount of physical differentiation 

 of the different layers of tissue, such as occurs in erect leafy shoots, stout leaf-stalks, 

 and tendrils, appears generally to be favourable to the production of the tensions in 

 tissues of which we have been speaking, as they are not found in stems of very 

 slow growth, like stout rhizomes, the thick stolons of Yucca and Dracaena, &c* 

 That the existence of tension has more to do with a physical differentiation in the 

 elasticity and extensibihty of the layers than with a morphological one, is shown by 

 the fact that very considerable tensions are found even between the outer and inner 

 layers of the hyphal tissue of the stems of the larger Hymenomycetous Fungi, which 

 are morphologically similar. Within the growing apical region of roots, on the con- 

 trary, where we have a combination of two layers of tissue sharply differentiated 

 morphologically, viz. an axial fibro-vascular bundle surrounded by a parenchymatous 

 cortex, we do not find any considarable tension when the part is split by two longi- 

 tudinal cuts at right angles to one another, or when the layers are completely isolated. 

 But since it is easy to prove that the cortex of the root grows more rapidly and for 

 a longer time than the axial bundle ^ it may be assumed that in an uninjured grow- 

 ing root there is nevertheless a small tension between them, positive in the case 

 of the cortex, negative in that of the axial bundle ; but it is only rarely that this 

 tension becomes strong enough to be perceptible by the parts bending inwards when 

 cut lengthwise ; probably because the axial bundle, although entirely composed of 

 procambial tissue, is so extensible that it yields almost without resistance to the 

 traction of the cortex. The case is different in the older parts of the root behind 

 the growing end (which does not exceed lo mm. in length). If this portion is split, 

 the parts generally gape concavely outwards, although much less so than the grow- 

 ing part of erect stems. The curvature is however considerable in the aerial roots 

 of Aroideae, where the opposite curvature which takes place at the apex is also 

 sometimes well-marked. 



The description now given of the states of tension in the case of stems is 

 also applicable to all expanded internodes and leaf-stalks. Within the bud itself, and 

 especially at the punctum vcgetationis, there appears to be no tension of the tissues, or 

 only one as slight as in the apices of roots. It is only when the epidermis is becom- 

 ing cuticularised and the walls of the bast-cells are beginning to thicken that the 

 tensions become perceptible. 



The individual parts of fully mature organs, especially leaves, not unfrequently 

 retain the tensions acquired during growth, which are in such cases often particu- 

 larly strong. This is the case, for instance, in the contractile organs of the sensitive 

 or periodically motile leaves of Papilionacese, Mimoseae, Oxalidese, &c., to which we 

 shall recur. While in these cases the true leaf-stalks and the internodes from which 



' The halves of roots split lengthwise continue lo grow for days, and bend concavely on ihe cut 

 surface. 



