PHYSIOLOGY 181 



actively AND involuntarily, as a sponge, but through the peculiar 

 selective power of their cells (p. 177) they exercise a choice from 

 among the substances available. 



The simpler and less highly developed plants, which are but slightly 

 differentiated, are able to absorb water through the surface of their 

 whole body. This is also generally true of all submerged aquatic plants, 

 even of the Phanerogams. Water-plants which obtain their nourish- 

 ment in this way often either possess no roots ( Utrimlaria, Salvinia), 

 or their roots serve merely as mechanical hold-fasts. With plants 

 living on dry land the conditions are quite different ; their stems 

 and leaves develop in the air, and they are restricted to the water held 

 by capillarity in the soil. In order to obtain this water in sufficient 

 quantities, special organs are necessary, which may spread themselves 

 out in the soil in their search for water. These organs must absorb 

 the water from the soil, and then force it to the aerial portions of the 

 plant. This office is performed for a land plant by its root system, 

 which, in addition to providing the supply of water, has also the task 

 of mechanically sustaining the plant, and withstanding all influences 

 which could lead to a disturbance of equilibrium by loosening the hold 

 of the plant on the earth. 



Conversely, loose soil is naturally bound together by the branching roots ; and 

 on this account plants have an economic value in holding together loose earth, 

 particularly on dykes and land subject to inundation. 



If the development of the root system of a germinating Bean or 

 Oak be observed, it will be found that the growing root of the embryo 

 at once penetrates the soil and pushes straight downwards. Lateral 

 roots are then given off from the main axis, and, growing either 

 horizontally or diagonally downwards, penetrate the earth in the 

 neighbourhood of the primary root. These lateral secondary roots in 

 turn develop other roots, which radiate in all directions from them, 

 and so occupy and utilise the entire soil at their disposal. The 

 branching of the root system can proceed in this manner until, within 

 the whole region occupied by the roots of a large plant, there is not a 

 single cubic centimetre of earth which is not penetrated and exhausted 

 by them. 



All plants do not form a deep-growing tap-root like that of the Oak, Silver Fir, 

 Beet, Lucerne, etc. ; some confine themselves to utilising the superficial layers of 

 the soil by means of a thickly-branched lateral root system (Pine, Cereals). The 

 agriculturist and forester must, accordingly, take into consideration the mode of 

 branching and growth of the roots of a plant just as much as the habit of growth of 

 its aerial portions. Plants which make use of different layers of soil may be 

 safely cultivated together in the same soil, and succeed one another in the same 

 ground. For similar reasons, in setting out trees along the borders of fields, the 

 deep-rooted Elm should be preferred to the Poplar, whose roots spread out near the 

 surface. 



Gardeners are in the habit of cutting off the tap-roots for the sake of conveni- 



