79^5 MECHANICS OF GROWTH, 



drawn from it. No increase of turgidity can therefore take place in this case, and 

 still less when the cell also increases in size. The same argument of course applies 

 also to a multicellular mass of tissue. But the case is different when the water with- 

 drawn from the cell-contents by the cell-wall is replaced by means of endosmose, 

 and the turgidity thus again increased ; in this case in proportion as water is absorbed 

 by the cell-wall the turgidity and volume of the whole cell must also increase. 



B. Mu/uai Tension of the layers of tissue of an organ, (i) Tension in the direction 

 of length; i.e. parallel to the axis of growth of the organ. In the internodes of 

 upright stems some idea may be obtained, if not of the intensity of the tension, at 

 least of its kind (whether negative or positive), and of its variation in the different 

 layers of tissue, by measuring the length of the internodes, and then separating the 

 layers of tissue by a sharp knife, and comparing their length with 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 what 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 tensioil 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 isolated strips of the epidermis, of the cortex, or of the wood (xylem), are shorter 

 than the entire internode, 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 the 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 E, the cortical 

 layer by C, the xylem by X, 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 was in a state of passive tension ; the pith 

 alone was 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, 



during imbibition indicates that a decrease of volume is taking place, and therefore that although v 

 is the amount of water absorbed by imbibition, the increase of volume is only v — d. 



