CONDITIONS AND MECHANICS OF GROWTH. 511 



on in the living cells, and the manifold changes in the production of organic com- 

 pounds of the food taken in from outside, and in the preparation of these com- 

 pounds for building materials. But they are not directly concerned in the insertion 

 and fixing of the building materials in the living cell-body, in the further growth 

 of protoplasm, and in the increasing dimensions of the growing cells, which last- 

 named processes alone may be looked upon as growth. Exactly how far water is 

 concerned in growth will be described in the following lines. 



Although only very little is known with regard to the minute structure of the 

 protoplasm of the cell, yet this much is beyond question, that it consists of firmer 

 and softer parts, which form an extremely complicated net-work, ever varying in 

 structure from species to species, and with meshes filled with very many different 

 substances, with water, fluid carbohydrates, albuminous compounds, dissolved salts 

 &c. It may also be imagined that fluid substances can be interpolated in the net 

 work, resembling it in structure and in consistency at the moment of insertion, 

 that is to say, which receive the same molecular arrangement, and so become an 

 organized portion of the cell-body. The cell-wall also, at the periphery of the 

 protoplasm, must possess a structure which renders it possible that between the 

 already-formed firm portions fluid molecules can be inserted, which then assume 

 the properties of those established portions. This insertion, however, presupposes 

 an extension of the firm parts already present, a separation of the groups of 

 molecules of the organized structures, and a place for the particles to be inserted, 

 and, on the other hand, repelling and attractive forces which control the portions 

 to be introduced. 



We are now prepared to admit that here a very important part must be 

 assigned to the turgidity of cells. As has been shown, the cell-sap of growing 

 cells is acid, and the acids and acid salts contained in it attract water from their 

 surroundings with considerable energy. The water thus brought into the vacuoles 

 of the protoplasm exercises a strong pressure on the peripheral layer, and indeed 

 on the cell-wall as well as on the protoplasm, which pressure first of all causes an 

 extension of these layers beyond the normal cohesive limit. By the elasticity of ' 

 the extended layers obviously a pressure is exercised on the fluid in the interior, 

 and this condition of mutual tension is called turgidity. In order to explain the 

 existence of this turgidity, it must be taken for granted that the water conveyed 

 into the vacuoles of the protoplasm by the attraction of the acids and acid salts 

 does not go back again, in spite of the pressure it exercises on the surrounding 

 layers; that it rather is held fast by the molecules of sugar and albumin in the 

 protoplasm. Experience conflrms this supposition, and it is evident that water 

 penetrates with great energy from the surroundings into the cells, that the cell 

 swells, the peripheral cell-layers experience a tension, and that yet no water pro- 

 ceeds through them. When protoplasm forces out water in consequence of a 

 stimulation, or when the strained layers are artificially punctured, only then does 

 the fluid come out of the rent formed like a tiny spring. But this again only 

 shows that the fluid in the interior is subject to a strong contra-pressure from the 



