PHENOMENA DUE TO THE TENSION OF TISSUES. 



799 



Silphium perfoliatum. 



Left half 



Right half 

 Silphium perfoliatum. 



Left half 



Right half 

 Macleya cordata. 



Hollow 



Length of 

 the entire 

 internode. 



69*5 mm. 

 69-5 



190 

 190 



Radius of 

 curvature of 

 the segment. 



4 cm. 

 4 



134*5 



3—4 

 3—4 



5-6 



Shorten- 

 ing of the 

 concave 

 outer (epider- 

 mis) side. 



2*8 p.C. 

 2-4 



2-8 

 2-6 



074 



Lengthen- 

 ing of the 



convex 



inner (pith) 



side. 



9'3p.c. 

 93 



9*5 

 10-8 



71 



Semidiameter 



of the 



internode. 



3 mm. 

 3 



35 

 45 



3*3 



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 a slice is somewhat longer than the entire internode, if the length of the 

 slice were taken as =100, the proportionate contraction 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 Draccena, &c. 

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

 elasticity and extensibility 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 stipes 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 cylinder surrounded by a parenchymatous 

 cortex, we do not find any considerable 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 cylinder^, 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 cylinder, which still consists of pro- 

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

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

 growing end (which does not exceed 10 mm. in length). If this portion is split, the 

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

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



* The halves of roots split lengthwise continue to grow for days, and bend concavely on the cut 

 surface. 



