8C0 MECHANICS OF GROWTH. 



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 vegeiaiionis , 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 be- 

 coming 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 un frequently 

 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 

 and periodically motile leaves of Papilionaceae, Mimoseae, Oxalidese, &c., to which we 

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

 they spring have long become rigid, and no longer show any considerable tension of 

 the tissues, an extraordinary elongation of the parenchymatous cortex occurs in 

 the contractile organs, if they are separated from the solid axial fibro- vascular 

 bundles; and considerable flexion results when these organs are split lengthwise. 

 The opposite to this occurs in the nodes of the stems of Grasses, i. e. in the annular 

 thickenings at the base of the leaf-sheaths ; no perceptible tension is observable in 

 these. If a median longitudinal section is made and divided into its inner and 

 outer layers, they exhibit none of the curvatures which are so striking in portions of 

 young internodes. This flaccidity of the tissue, or at least the insignificance of the 

 tension, must depend on the concurrence of two causes ; on the one hand on the 

 cessation of the growth of the parenchyma in the node (although it remains in a 

 state capable of growing, and under certain circumstances begins to grow again), 

 and on the other hand on the extensibility of the fibro-vascular bundles which do 

 not become lignified within the node, or not till a late period when the cells of the 

 same bundles, where they lie in the leaf-sheath and the internode, have long become 

 lignified and rigid. While, therefore, the parenchyma of the node continues to 

 grow, it stretches the unresisting fibro-vascular bundles, and when its growth ceases 

 no perceptible tension remains. In the contractile organs of sensitive and periodi- 

 cally motile leaves, on the contrary, the axial fibro-vascular bundle becomes elastic 

 and resistant before the growth of the surrounding parenchyma has ceased; and 

 when this is the case a tension remains which is further increased by the extra- 

 ordinary capacity of the parenchyma for becoming turgid. 



If we now attempt to give an account of the causes which render the tension 

 at first (when in the bud) imperceptible in the internodes of erect rapidly-growing 

 stems, and make it subsequently increase and finally altogether disappear when the 

 internodes are fully mature, we find that we must content ourselves with probable 

 conjectures rather than with fully demonstrated propositions. 



The origin of tension between the layers must in any case be referred mainly 

 to differences in the growth of the cell-walls of such a nature that the intercalation of 

 fresh material takes place less rapidly in those of one layer than in those of another ; 

 and it is especially manifest in those cases in which the cell-walls subsequently undergo 

 thickening. From the first of these causes the layers which lengthen more slowly are 

 placed in a state of passive tension by those that grow more rapidly; while the second 



