PHEXOMENA DUE TO THE TEXSION OF TISSU7.S. 72 1 



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 

 the 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 rate of growth of the cell-walls, it may arise from the inter- 

 calation of fresh material taking place less rapidly in one layer than in another ; and 

 it is especially manifest when the cell-walls in the one case 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 cause diminishes their extensibility to an increasing extent, especially when, 

 as in the xylem of the fibro-vascular bundles, the cell-walls become lignified, which 

 renders them capable of resisting extension. The more quickly, on the other hand, 

 the thin cell-walls in the pith and parenchyma generally increase in size (especially 

 in length) by superficial growth, the stronger becomes the tension of the passively 

 distended layers of tissue. To this must be added the peculiar power of the medul- 

 lary cells to absorb water from the older parts with great force and rapidity, and thus 

 to maintain themselves in a state of the highest turgidity. This distends the pith 

 independently of the superficial growth of its cell-walls, and besides influencing the 

 more slowly growing layers of tissue, also contributes to increase the superficial 

 growth of the cell-walls of the pith. If the woody bundles then become lignified as 

 the tissues become more developed internally, and the resistance of the epidermis, 

 which is constantly becoming more cuticularised, becomes too great, these tissues 



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