778 
MECHANICS OF GROWTH, 
sprung up successively in the course of time, /. e. in a series of generations. Tlie 
chief evidence in favour of this view will be given in the last chapter of this work. 
It need only be mentioned now that this theory of the genesis of specific properties 
indicates the only possibility of arriving at an understanding of them in accordance 
with the laws of causality. At the present time this is possible only in the most 
general outline. 
The use here made of the terms 'historical' and 'physical' may also be illustrated 
from another subject in the following manner. The nature of the geological form- 
ations of which the crust of the earth consists can be understood only from a 
historical point of view, because it is only at particular spots and at particular times 
that the conditions have concurred which produced, for example, the Chalk or the Old 
Red Sandstone. The formation of these rocks was dependent on chemical and 
physical processes, which must however have been preceded by other physical 
changes in the crust of the earth, in order that these rocks should be formed exactly 
at particular spots and particular periods. A crystal of sodium chloride can, on the 
contrary, be produced at any time, for the necessary conditions may be artificially 
brought together. Pseudomorphism of crystals can again be explained only from a 
historical point of view, although it is certain that the chemical and physical properties 
of the substances are alone concerned in the process. 
We see therefore — and this is the object of these remarks — that the historical 
explanation of a natural phenomenon does not exclude its explanation from a physical 
point of view, but on the contrary includes it where we have to do with natural 
phenomena; and this principle is equally applicable to those properties of vegetable 
species which have been acquired hereditarily or historically, even when the application 
is practically much more difficult than in the case of inorganic nature. 
Sect. 13. General Properties of the Growing Parts of Plants ^ From 
the consideration of this subject the true crystals which are found in cells may be 
entirely excluded, since they do not differ in their general properties from those 
which occur elsewhere. The organised elementary structures on the contrary, the 
protoplasm, the nucleus, chlorophyll-granules, starch-grains, and the cell-walls, ex- 
hibit properties which distinguish them from all unorganised bodies. 
These organised bodies are, in the first place, all capable of swelling; i.e. 
they have the power of absorbing water or aqueous solutions between their solid 
particles with such force that the particles are forced apart ; the whole structure 
increases in size, and can thus exercise considerable pressure on the surrounding 
parts. If water is by any means withdrawn from the body which has thus swollen 
up, its particles again approach one another, and with such force that considerable 
strains may be exerted on the adjoining parts connected with it; as, for example, is 
shown in the bursting of dry capsules. The swelling and desiccation of organised 
parts may therefore cause change of form in the surrounding parts, i.e. in other 
organised parts. This power of swelhng is of still greater importance, since it is 
this process that renders possible the interchange of sap between the individual cells 
as well as between whole masses of tissue. In order that growth by intussusception 
may take place, the dissolved food-materials must be able to enter by imbibition be- 
tween the particles of the growing structure, and the chemical processes must take 
place there which construct from the dissolved food-materials solid particles to be 
intercalated between those already in existence, and in consequence of which the 
organic mass alters its volume and form (see Book III. Sect. i). 
^ See Nägeli u. Schwendener, Das Mikroskop, p. 540 et seq. 
