4i 



the purpose of absorption, and no particular organs are necessary. 

 Further, everything the plant takes up for its nutrition, even 

 the Carbonic Dioxide gas, is derived from the water which sur- 

 rounds it, and consequently must be in solution in water. What 

 is taken up by such a plant, then, is taken up in obedience to 

 the laws of diffusion, modified by the controlling influence 

 exerted by the protoplasmic layer lining the cell-wall ; and this 

 is true of the great majority of plants — viz., that absorption 

 takes place through the cell-wall. In some cases, however, we 

 find that no cell-wall is present, but simply a mass of proto- 

 plasm — e.g., in the Myxomycetes — and in this case the process 

 is somewhat different, the protoplasm simply flowing round the 

 solid particles ; and then these particles are not absorbed into 

 the protoplasmic substance, but are merely enclosed in it, then 

 the protoplasm acts upon them, bringing those which are fit for 

 food into solution, and dispersing them through its mass, while 

 those which are unfit for digestion are afterwards thrown out. 

 This is an exceptional case, however, for the cell-wall usually 

 prevents the direct contact of the protoplasm with the food. If 

 there was no living protoplasm lining the cell-wall, the process 

 of absorption would be a simple process of absorption through 

 a membrane, in obedience to the laws of osmosis, for if we had 

 simply a cell-membrane, osmosis would go on freely and equally 

 in all directions, but the presence of the protoplasmic lining 

 removes it from the physical condition of things. Thus, if we 

 take a typical cell, the amount per cent, of salts taken up is 

 very much less in the cell-sap than outside, if the cell be 

 immersed in fluid. That is to say, the cell takes up the water 

 in which the substances are dissolved more readily than it takes 

 up the substances themselves, consequently the more dilute the 

 solution is the greater is the amount of salts taken up. It would 

 seem, at first, to be a waste of energy on the part of the cell to 

 take up large quantities of a dilute solution, filter the water off, 

 and retain the salts, rather than to take up small quantities of a 

 more dense one ; but the fact is, that dense or strong solutions 

 are interfered with in their passage by the protoplasm lining 

 the cell- wall, and if they were sufficiently dense and strong they 



