788 
MECHANICS OF GROWTH. 
they imbibe or lose water ; and (3) the changes in volume and form caused by the 
growth of the cells. 
I . Turgidiiy. The force by which water is drawn by endosmotic attraction ^ 
to the cell from the parts that surround it is not merely sufficient to fill the space 
enclosed by the cell-wall, but also to enlarge it, the increasing amount of sap dis- 
tending the cell-wall until its elasticity is brought into equilibrium with the endos- 
motic absorption. In this condition the cell-wall is stretched to its full capacity, 
or the cell is turgid. If the cell loses a portion of its water by transpiration or by 
neighbouring cells withdrawing it, the tension of the cell-wall is decreased and the 
volume of the cell diminished. The hydrostatic pressure produced by the endosmotic 
action of the cell-wall acts from within and is the same at all points within the 
small cell-cavity; but this does not prevent different points of the cell-wall stretch- 
ing and contracting in different degrees as the turgidity increases, in consequence of 
local variations in extensibility. Hence not only may the volume but also the form 
of the cell be changed by turgidity. The greater the tension between the cell- 
wall and its contents, in other words the greater its turgidity, the greater is the 
resistance offered by the cell to external forces which tend to alter its form by 
pressure, but the more readily does it burst in consequence. If the cell loses so 
much water that the space enclosed by the flaccid cell-wall is no longer filled, it may 
become folded inwards by the external pressure of the air or of the surrounding 
water, and in this case the cell is said to collapse ; if the cell-wall is thick, firm, and 
inflexible, a tension of an opposite character to turgidity takes place in the cell. 
Since turgidity is nothing but the mutual tension of the cell-wall and contents, or 
a state of equilibrium between endosmotic absorption and the elasticity of the 
cell- wall, it is evident that only closed cells, i. e. such aa have no orifices, can be 
turgid. The micellar interstices through which the water set in motion by endos- 
mose forces its way into the cells are essentially different from pores ; the former are 
so small that their diameter is completely under the control of molecular forces, 
while even the smallest pore withdraws at least the middle portion of its space 
from the influence of the molecular action of the substance that bounds it. Micro- 
scopic openings, like the pores of bordered pits, are orifices of this latter kind, and 
are excessively large compared with the micellar interstices through which endosmose 
acts. Cells with pits penetrating the cell-wall cannot therefore be turgid, because 
any tension however small between cell-wall and contents is at once neutralised by 
the superfluous sap becoming pressed out through the orifices. It is indeed possible 
for water to be forced out in this way even through closed cell -walls, but only when 
the turgidity is very great, and the hydrostatic pressure of the cell-sap on the per- 
fectly tense cell-wall is sufficient to force out the water through the micellar inter- 
stices ^. The resistance offered by the cell-wall to this may be called resistance to 
^ [It appears probable that the organic acids which are present in the cell-sap of all cells which 
are or can be turgid are the substances which induce endosmosis (see de Vries, Ueb. die Bedeutung 
der Pflanzensäuren für den Turgor der Zellen, Bot. Zeitg. 1879). De Vries is of opinion (Bot. 
Zeitg. 1879, Ueb. die inneren Vorgänge bei den Wachsthumskrümmungen mehrzelliger Organe) 
that growth in length depends upon the continuous production of actively osmotic substances in the 
cell-sap of the growing cells.] 
2 That the water which filters through under such circumstances actually passes through 
