ABSORPTION OF FOOD, AND EXCRETION. 495 



this case the contents of the cells will vary from the absorbed 

 water much more than in the former ; for" the endosmotic attrac- 

 tion must overpower the resistance with which the water is held 

 in the soil. The most common and important medium in this case 

 is found in the decomposition of vegetable substances rich in car- 

 bon, which are known by the collective names of garden -soil, mould 

 (humus). It is often found also in inorganic substances, endowed 

 with similar physical properties. The greater or less facility with 

 which they are able to absorb and condense water, carbonic acid, 

 and salts of ammonia from the atmosphere, is important. For this 

 purpose mould is the best possible medium. The great aim of 

 culture should be to endow the earth as richly as possible with the 

 physical qualities requisite to serve the plants that are to grow 

 in it. 



The third case is that in which plants vegetate only in the air. 

 It has only recently been discovered with certainty, that this case 

 actually exists in the vegetation of the tropical Orchidacece. In 

 such plants, the root-sheath appears to supply the place of soil, and 

 they draw their nourishment from the surrounding air. 



In all these cases, the absorption of matter by endosrnosis is 

 doubtless connected with a process of excretion, though small. 

 Such excretion is produced by the endosmosis of the cell contents 

 and assimilated matter of the plant. The comparison of these with 

 excrements, as matter which the plant has worn out, is perfectly 

 inapplicable, and cannot be supported by accurate investigation. 



Up to the present time we know of no other power by which fluid can 

 penetrate into the interior of a cell, than that which occurs as the result 

 of mixing two fluids, separated by a permeable organic membrane, and 

 which is at the present time called endosmose. We cannot, therefore, 

 regard the act of absorption from any other point of view ; at the same 

 time, the observations upon endosmosis are only recent, and there are 

 yet many unanswered questions which can only be solved by accurate 

 observation, and all hasty generalisations and hypotheses must be 

 eschewed. The different localities in which plants grow have been well 

 known, but because nothing has been known with regard to the processes 

 of absorption, no distinction of these localities could be made according 

 to the characteristic modes of the absorption of nutriment. No sooner, 

 however, do we see that endosmosis lies at the basis of absorption, than 

 we discover that the above three distinctive modes of it demand atten- 

 tion. The simplest case in which plants are, for the most part, in con- 

 tact with water, occurs least frequently in the Phanerogamia, those 

 plants which have been almost exclusively regarded as the object of 

 physiology; and the results of endosmotic experiments have been applied 

 to these alone. If endosmotic phenomena are regarded, without taking 

 a wide view of the different kinds of vegetation as it occurs in the earth, 

 on stones, in wood, &c., which produce an essential difference in the 

 relation of water to the plant, only the most superficial knowledge can 

 be obtained. It is only by extended observations that we can expect to 

 explain the relations of this subject, and to fill up the hiatus in our 

 knowledge of the subject. Yet all previous vegetable physiologists who 

 have treated of this subject have supposed that a plant is growing in 



