OF THE BOOT OR DESCEXDIXG AXIS. 74o 



increased development of rootlets and fibrils takes place for its 

 absorption. 



Roots can only absorb substances in a liquid state, therefore 

 the different inorganic substances which are derived from the 

 soil, and which form an essential part of the food of plants 

 (page 793), must previously be dissolved in water. If the roots 

 of a freely growing plant be placed in water in which charcoal 

 in the most minute state of division has been put, as that sub- 

 stance is insoluble in the fluid, it will remain on the surface of 

 the roots, and the water alone will pass into them. 



Various experiments have been devised to ascertain whether 

 the plant possesses any power of selecting food by its roots. 

 Saussure proved, that when the roots of plants were put into 

 mixed solutions of various salts, some were taken up more freely 

 than others. He also found, that dead or diseased roots absorbed 

 differently to those in a living and healthy condition. The 

 experiments of Daubeny, Trinchinetti, and others lead essentially 

 to the same conclusions. Again, though the seeds of the common 

 beau and wheat be sown in the same soil, and exposed, as far as 

 possible, to the same influences in their after-growth and deve- 

 lopment, yet chemical analysis shows that the wheat stalk con- 

 tains a much larger proportion of silica (which it must have 

 obtained from the soil) than that of the bean. 



The experiments of Bouchardat, Vogel, and others appear, on 

 the contrary, to indicate that roots absorb all substances pre- 

 sented to them indifferently, and in equal proportions. The 

 simple fact, however, which is easily proved by chemical 

 analysis — that the ashes of different plants contain different 

 substances or in different proportions — seems to prove incon- 

 testably that roots have a power of selecting their food. In 

 using the term selecting, however, we do not intend to imply that 

 roots have any inherent vital power of selection resembling 

 animal volition, but only to express the result produced by virtue 

 of the mutual actions of the root and the substances which sur- 

 round it in the soil. This power or property of selection is 

 without doubt due to some at piresent but little understood 

 molecular relation which exists between the membranes of the 

 cells of different plants and the substances which are taken up or 

 rejected by them, different roots possessing different osmotic 

 action for the same substances. It follows also, from the recog- 

 nition of this action as the cause of the absorption of fluid 

 matters by the plant, that poisonous substances may be also 

 taken iip when in solution by the roots, provided their tissues 

 are not injured by them in their passage; and we find, accord- 

 ingly, that when such substances are found in the soil, a cor- 

 responding effect is produced upon plants by their absorption. 



Excretion hy Boots. — The roots of plants have been considered 

 by Brugmans, Pleuk, De Candolle, Macaire-Prinsep, and others, 



