698 MECHANICAL LAWS OF GROWTH. 



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 If water is by any means withdrawn from the body which has thus swollen 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 dessication of organised 

 parts may therefore cause change of form in the surrounding parts, i.e. in other 

 oro-anised 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 

 mav take place, the dissolved food-materials must be able to enter by imbibition be- 

 tween the part'cles 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 w'hich the 

 organic mass alters its volume and form (see Book III, Sect. i). 



A second general property of the organised parts of plants is that they change 

 thei)' form when the external conditions remain perfectly unaltered, internal changes 

 being the only efficient cause. Almost every process of growth is associated with 

 change of form. These facts may be more briefly described by ascribing to 

 organised structures endowed with the power of growth internal forces or plastic 

 tendencies, if it is clearly understood that the term is only used to express a still 

 unresolved aggregate of causes. As a result of these internal forces, organised 

 structures have the power of overcoming resistance. Thus, for example, plasmodia 

 which are constantly altering their form, are able, notwithstanding their gelatinous 

 and very soft nature, to overcome their own weight, and to creep up solid bodies. 

 In the same manner the growth of wood takes place with such force as to overcome 

 the very considerable pressure of the surrounding bark. 



But although the internal causes of these plastic tendencies are able to over- 

 come certain obstacles, it is on the other hand certain that growth is also influenced 

 by external forces, such as pressure, traction, stretching, bending, &c., which are 

 able to alter the form of solid bodies. The observations w^hich have been made on 

 this subject will be collected in the following sections ; but it is in the first place 

 necessary to define certain terms which will frequently be employed. 



Like unorganised €olid bodies, those which are organised oppose a greater 

 or less resistance to the external forces which tend to alter their form ; and are 

 hence divided into hard and soft bodies. A hard body is one which offers con- 

 siderable resistance, like many lignified or silicified cell-walls ; a soft body is one 

 which offers very little resistance, like protoplasm, chlorophyll-grains, or swollen cell- 

 walls which have ceased growing, as gum-tragacanth. Structures which become 

 disintegrated under pressure and traction rather than undergo any considerable 

 change of form, are brittle, like grains of starch or crystalloids of aleurone. If, on 

 the contrary, they are capable of undergoing considerable changes of form, whether 

 this take place by pressure or traction, they are extensible. It is clear that flexibility 

 depends to a certain extent on extensibility, since the side of the bent part which 

 becomes concave is compressed, the convex side stretched. All these properties are 

 relative, and the sam^e body may exhibit different phenomena according to the 



