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. Mutual 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 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 Httle 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. 
Ail 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 the cortical 
layer by C, the xylem by 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. 
