PHENOMENA DUE TO THE TENSION OF TISSUES. Jiy 



of tissue by a sharp knife, and comparing their length \vith that of the entire 

 internode. It is obvious that the length of the entire internode is the result of 

 the mutual tensions of its layers, some being, in this experiment, shorter and some 

 longer than the entire internode ; and it results from M'hat has already been said 

 about opposite tensions that if any particular layers have not changed in length 

 after being separated, this does not prove that they were not distended or com- 

 pressed when forming a part of the system, but only that they opposed a strong 

 resistance to the tension then in existence, which resistance rendered the alteration 

 of their length imperceptibly small. But the opposite is also possible ; viz. that a 

 layer of tissue when separated will show no perceptible contraction because it was so 

 extremely extensible and inelastic that it yielded with extremely little resistance to 

 the traction of the layers which were in a state of positive tension, the limit of its 

 elasticity being continually overstepped. 



If this method is applied to rapidly growing internodes, it is generally found 

 that the epidermis, the bark, or the wood (xylem), are shorter than the entire inter- 

 node, while the isolated pith is considerably longer; the former therefore were in 

 a state of negative, the latter was in one of positive tension. All isolated layers are 

 flaccid, while the entire internode was rigid from the mutual tension. 



If a median longitudinal lamella bounded by two strips of epidermis is cut 

 out of a growing internode with its xylem still unlignified, and if its tissues are 

 then isolated so as to lie side by side, then, indicating the epidermis by J^, the cor- 

 tical layer by C, the xylem by A', the pith by P, the respective lengths after isolation 

 may be stated as follows : — 



E<C<X<P>X>C>E. 



It is at once evident from this that every layer was before the separation in a state 

 of negative tension towards the next one inside, of positive tension towards the 

 next one outside. The epidermis alone i.j in a state of passive tension ; the pith 

 alone is passively compressed, or rather prevented from extending. 



The extensibility and elasticity of tissues are altered during the growth of an 

 internode, as may be seen by comparing internodes of various ages ; the exten- 

 sibility of the wood decreases rapidly, that of the epidermis and cortex more slowly, 

 as may be inferred from the decreasing rapidity with which these tissues contract on 

 their isolation, and from the thickening of the cell-walls ^ The pith from internodes 

 of different ages shows on isolation at first an increasing, afterwards a decreasing- 

 amount of elongation. If the tendency of the pith to expand remained the same 

 at all ages, it would, when isolated, elongate more in older than in younger inter- 

 nodes, in consequence of the increasing resistance of the tissues which are in a 

 state of passive tension ; but when the growth in length has ceased, or soon after, 

 the pith loses its tendency to expand, as may be concluded from the fact that on 

 isolation from such internodes it elongates less, and finally not at all^, although the 



^ The decrease in the extensibility of the epidermis was determined by Kraus (/. c, tables, 

 p. 9), by attaching weights to strips of epidermis. 



2 The relation between the tension of tissues and the state of growth of the internode (2. e. 

 the phase of its greatest period of growth) requires fresh and detailed investigation. Kraus's 

 Table III (Bot. Zeitg. 1867), shows that the greatest difference of length between cortex and pith 

 does not always occur at the time of the greatest growth ; and that even after growtli has ceased, 



