774 
MECHANICS OF GROWTH. 
processes, taking for granted at the outset the purely formal phenomena of mor- 
phology, and regarding as the ultimate object of the enquiry the obtaining an 
insight into the mechanism of growth. If the solution of this difficult problem 
must be deferred to a distant future, it at any rate lies within the scope of this 
work to collect together the ascertained phenomena. But even here we meet 
with the difficulty that no one has as yet undertaken to limit the term Growth to 
a definitely circumscribed idea. The term is however always employed in the case 
of plants and animals to designate permanent changes in form or volume or both, 
brought about by internal causes, themselves the result of organisation, and in their 
turn excited and maintained by definite external causes, as heat, light, gravitation, 
the supply of food-materials, water, &c. Changes in the form or volume of parts 
of plants that remain quite passive to external forces, and in which changes no 
organising process cooperates, ought not to be included in the term Growth. Thus, 
for example, there is no growth when the form or length of an internode or root 
is altered by simple stretching, pressure, twisting, or bending (it may be by the 
hands). It is quite possible however that under certain circumstances internal 
changes might be brought about by external influences to which the part of the 
plant is at first altogether passive, but which, combined with organising processes, 
cause true growth or changes of growth. By organising processes I understand those 
internal changes which fulfil the two following conditions : — firstly, they are caused 
by the specific organisation of the part of the plant, which is of such a nature that 
any external influence can only efl"ect changes in accordance with it; secondly, 
they result in a permanent change of the organised part which is not at once 
reversed by opposite external influences. If, for example, the elevation of the 
temperature above the inferior limit (see Sect. 7) has caused an increase in volume 
of the embryonic structures already saturated with water, the parts will not contract 
to their previous volume when the temperature again falls below this point, but will 
retain the increase acquired during the higher temperature ; in other words the 
process is not reversed, it only ceases. Microscopic as well as other kinds of ex- 
amination also show that the internal organisation has undergone permanent change 
varying with the specific properties of the plant. If on the contrary a stem is 
allowed to wither from want of water, it becomes shorter and ceases to grow ; when 
it again absorbs water it becomes longer and thicker and begins to grow. The 
contraction on withering and the lengthening on the absorption of water are mere 
physical phenomena ; but the lengthening and thickening of a part resulting from 
continued turgescence may actually depend upon growth, the organisation of the 
plant being altered permanently and to an amount varying with the species by the 
operation of the turgescence. It is again the result of permanent and specific 
change of organisation when a tendril, in consequence of the light pressure of the 
body to which it clings, lengthens less on the side in contact, more on the opposite 
exposed side ; the curvature thus caused does not disappear if the pressure has 
lasted long enough ; the whole phenomenon is therefore one of growth. When, on 
the contrary, the motile organ of a MimosaAtdii bends downwards in consequence 
of irritation, and afterwards again bends upwards, this is, it is true, caused by the 
peculiar organisation of the plant; but the movement induces no change in the 
organisation itself, and its efi"ects are not permanent, the leaf soon returning to 
