182 BOTANY part i 



in the Myxomycetes, the naked protoplasm is able to surround and 

 thus to absorb solid particles. 



II. Absorption and Movement of Substances 



A. Absorption of Substances into the Cell('') 



We may best commence our consideration of the absorption of 

 substances with the free-living cell, and investigate in what manner 

 this absorbs water, solid substances, and gases. 



Water. — All parts of a plant and all the parts of its individual 

 cells are saturated with water. The cell-membrane has the water so 

 freely divided between its minute particles that the water and the 

 solid substance are not distinguishable under the highest magnification. 

 If the water is allowed to evaporate, air-filled cavities do not appear 

 in its place, but a contraction of the cell-wall takes place. On the 

 other hand, the absorption of water by dry or not fully saturated cell- 

 walls causes a swelling of the latter ; this takes place with consider; 

 able energy, and can overcome considerable resistance. The walls of 

 lignified cells absorb about one-third of their weight of water. The i| 



walls of some Algae and of the cells of some seed-coats and fruits [ 



consist more largely of water than of solid substance. The behaviour 

 of the protoplasm is similar. Both wall and protoplasm are 

 capable of swelling, and in the actively living cell are always in 

 this condition. So long as the protoplasm is not saturated with water, 

 it removes water from the cell-wall ; owing to this, the wall which 

 may before have been saturated becomes poorer in water, and seeks to 

 replace the loss by taking up water from its surroundings. In this 

 way the loss of water by the protoplasm affects the outer world by 

 the medium of the cell-wall, 



AYith the cell-sap it is different. This is a solution of various 

 organic and inorganic substances in water. For simplicity we may 

 assume that we are dealing with a solution of salts. If such a 

 solution is enclosed by a cell-membrane, if we, for instance, fill a tube 

 of cellulose with the solution and fasten the ends firmly, and then this 

 "cell" is placed in water, the salts will diffuse into the water and 

 water will diffuse into the cell ; this goes on until the same concentra- 

 tion is attained at all points both within and without the cell. A 

 partition which is equally permeable to water and salts has no effect 

 on the movements of diffusion which take place in every free 

 mass of fluid ; everything goes on as if the partition were not 

 present. If this cell-membrane is rejolaced by one, which, while | 



being permeable to both water and salts, lets the water pass 

 through much more rapidly than the salts, more water will at first 

 pass into the cell than salt passes out ; thus a pressure will arise 

 within the cell expressed, in an increase of the volume of the cell, and in 



