778 lifECHANICS OF GROWTH. 



sprung up successively in the course of time, /'. e. in a series of generations. The 

 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 sodiuni 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 tlie 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 swelling 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 Nageli u. Schwendener, Das Mikroskop, p. 540 et seq. 



