PHENOMENA DUE TO THE TENSION OF TISSUES. 
799 
Shorten- 
Lengthen- 
Length of 
Radius of 
ing of the 
ing of the 
Semidiamel* 
cu.rvatu.rc of 
pnnm VP 
\^\J LL\^<X, V v.- 
convex 
of the 
internode. 
the segment. 
outer (epider- 
mis) side. 
inner (pith) 
side. 
internode. 
Silphium perfoliatum. 
Left half 
69*5 mm. 
4 cm. 
2*8 p.c. 
9-3 p.c. 
3 mm. 
Right half 
69*5 
4 
24 
9*3 
3 
Silphium perfoliatum. 
Left half 
190 
3—4 
2-8 
9-5 
3'5 
Right half 
190 
3—4 
2-6 
10-8 
45 
Macleya cordata. 
Hollow 
i34'5 
5-6 
074 
71 
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 dififerent 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 aerial roots 
^ The halves of roots split lengthwise continue to grow for days, and bend concavely on the cut 
surface. 
