CHAPTER II 



THE MECHANICS OF GROWTH 



SECTION 7. General. 



IN a general study of the mechanics of growth we can confine 

 ourselves to the individual cell, for the growth of a tissue is simply the 

 result of the correlated growth of the cells of which it is composed. 

 In the case of a dermatoplast, the cell-wall must grow, if any external 

 change of shape is to be produced. The growth of the cell-wall is a 

 vital phenomenon rendered possible by protoplasmic activity. For the 

 continuance of growth, the proper performance and regulation of certain 

 other functions are directly or indirectly necessary, while as the result 

 of the growth of the cell the mass of protoplasm may increase, and 

 a division of it and of the nucleus may be induced. At the same time 

 the turgor is maintained by a sufficient production of soluble metabolic 

 products. Similarly the cessation of the production of starch beyond a 

 certain limit, and the formation of a new wall around a plasmolysed 

 protoplast, afford good instances of automatically regulated growth. 



It must, however, be remembered that growth is simply a special 

 form of protoplasmic activity, and hence the study of this special 

 function ultimately involves that of vital activity in general, for the 

 internal factors concerned in growth are themselves vital complexes. 

 Even if the mode of growth of the cell-wall were completely understood, 

 we should have solved the mechanism of a particular partial function, 

 but not that of growth in general. Indeed, in gymnoplasts which possess 

 no cell-wall, we are at once confronted with the fundamental problem 

 of protoplasmic growth. In this direction our knowledge is at present 

 extremely deficient, and hence a few general remarks must suffice. More 

 is known as to the mode of growth of the cell-wall and of the starch-grains, 

 but the growth of crystals and crystalloids is a purely physical problem 

 and need not be discussed here. 



The power of imbibition and of swelling plays a most important 

 part in metabolism, and hence in the liberation of energy as well as in 

 the production of plastic material. Independently of where the particles 

 of plastic material are finally deposited, their power of penetrating between 

 the particles of pre-existent materials and causing growth by intus- 



