24 METABOLISM 



DEVRIES, H. 1884 A. Bot. Ztg. 46, 229. 

 DE VRIES, H. 1888 B. Ibid. 46, 393. 

 DE VRIES, H. 1889. Ibid. 47, 309. 



[A complete exposition of osmosis and related phenomena may be found in 

 B. E. LIVINGSTON'S treatise, The Role of Diffusion and Osmotic Pressure in Plants. 

 Chicago. 1903. See also HOBER, 1902, Physikalische Chemie der Zelle und der 

 Gewebe. Leipzig.] 



LECTURE III 



THE ABSORPTION OF WATER 



LEAVING now the consideration of the simple relationships of the single cell 

 which, when surrounded by water on all sides, is able to absorb water with 

 dissolved gases and solids, dependent on the degree of permeability of the 

 protoplasm, we have next to inquire into the mode of absorption of materials 

 by the complex plant body. Examination of such a cell-complex as we meet 

 with in the higher Algae (Florideae, Fucaceae) or in a submerged or floating 

 Phanerogam (e. g. Lemna triscula), discloses a superficial cell layer whose 

 individual units are, so far as absorption of materials from without is concerned, 

 in all respects comparable to the solitary independent cells of which we have 

 already spoken. Beneath the superficial cells, however, we find internal cells 

 which are prevented from obtaining direct access to the external medium, 

 and are dependent on such materials as have been permitted to pass through 

 the cell-walls and protoplasm of the more peripheral cells. The external cell 

 layer, in the first instance, determines what substances shall pass into the 

 internal cells, although not all substances capable of entering the superficial 

 layer necessarily pass farther inwards. The internal cells may in a sense be 

 considered to be in direct relation with the external medium, since they are in 

 connexion with it through their cell-walls, in which, generally speaking, all 

 the substances in question more especially water can move. 



In principle this is true of all land plants, and one might affirm that a 

 cell in the topmost bud or leaf of an oak-tree was in direct communication, 

 through the cell-walls, with the aqueous solution in the soil in which the roots 

 are imbedded, although thousands, or even millions of cells intervene between 

 it and the tips of the roots. In practice, however, this case is quite different 

 from the preceding one, since further exposition will show that any exchange 

 of materials is impossible by this means, owing to the enormous distance which 

 separates the units. Hence we must regard the absorption of materials in the 

 land plant as quite distinct in character from that occurring in a single sub- 

 merged cell. No physiological researches are requisite to prove that the two 

 regions of the terrestrial plant, manifestly different even to the non-botanist 

 viz. the root imbedded in the soil and the shoot expanded in the air, are essentially 

 different in their methods of absorbing nutriment. The root absorbs the water 

 present in the soil and the substances dissolved in it in the way already 

 described, while the shoot absorbs the materials in the air in the gaseous 

 form essentially. Naturally, therefore, in the following pages we must con- 

 sider separately those constituents of the higher plant which are derived from 

 the soil and those which are absorbed from the air. 



First of all the plant absorbs water from the soil, which, as is well known, 

 is indispensable to all organisms and to the vegetable kingdom in particular. 

 Apart altogether from the fact that the chemical elements which go to form 

 water, i. e. oxygen and hydrogen, form, in combination with carbon, the most 

 important constructive units in organic compounds, water itself is an indispen- 

 sable constituent of all cell membranes, which, as a matter of fact, are, in the 



