224 BOTANY PART i 



of cellulose is filled with a solution, for instance of common salt, and 

 placed in water, a process of DIFFUSION will commence. Water 

 passes into the tube while salt passes out from it. Although the wall 

 of the cell offers greater resistance to the passage of the salt than of 

 the water, the diffusion if continued long enough will result in the 

 same concentration being attained at all points both within and 

 without the cell. A partition which is permeable to both water and 

 salts thus only affects the process of diffusion by diminishing its 

 rapidity. When the wall consists of a substance which is readily 

 permeable to water but quite impermeable to the salt, the course of 

 diffusion is essentially different. If such a SEMI-PERMEABLE MEM- 

 BRANE is employed, there is no question of a diffusion of the salt, but 

 the conditions permit of a diffusion of water 

 inwards. Since within the semi -permeable 

 membrane a portion of the space is occupied 

 by the molecules of .salt, the water is here less 

 concentrated than outside. A diffusion from 

 the more concentrated to the less concentrated 

 -^ water, therefore, takes place. Such a one-sided 

 diffusion is termed OSMOSIS ( 9 ), and it results in 

 a condition of pressure (OSMOTIC PRESSURE) 

 within the cell. 



A physical apparatus may, in the first 

 instance, be employed to demonstrate and 

 measure the osmotic pressure. Since semi- 

 Fio.m-osmometer.r.ciay permeable membranes are mostly delicate, they 



cell with the precipitation l TIT 



membrane (N) ; B, mano- are Supported by a Solid but pOrOUS Substratum ; 



meter with mercury (Q) -, they may be deposited on the walls of cells of 



z, sugar solution, unglazed clay. Such a cell (Fig. 236) may, 



for instance, have a semi-permeable membrane 



of ferrocyanide of copper deposited on its inside. The cell is then 

 filled with a solution of sugar, closed, provided with a m.ercury 

 manometer, and immersed in water. The osmotic pressure is indicated 

 by the rise in height of the mercury. It has been found that a 

 1 per cent solution of cane sugar can give rise to a pressure of f atmo- 

 sphere. Assuming that the semi-permeable membrane is impermeable 

 to the dissolved substance, the effect of all solutions of crystalloids is 

 nearly proportional to the number of molecules and ions present. 

 Solutions that produce the same osmotic pressure are termed isosmotic ; 

 thus, for example, 0'58 per cent NaCl, 27 per cent grape sugar, and 

 5'13 per cent cane sugar, are isosmotic with 1 per cent potassium 

 nitrate. 



The clay cell corresponds to the cell wall and the ferrocyanide of 

 copper membrane to the protoplasm. In the vegetable cell itself the 

 cell wall is completely permeable apart from some special cases ( 9a ). 

 The layer of protoplasm applied to it, on the other hand, is more or 



