604 



tion and assimilation, to render them fit materials for development 

 and secretion. The term nutrition does not appear to be applicable 

 to any process in vegetable life. Unlike animal organisms, where 

 absorption is continually removing effete structures, to be replaced 

 by the nutritive powers, the organs of plants are produced by deve- 

 lopment, which continues up to a certain point ; the organ after this 

 merely acts in consequence of mechanical structure, and when effete 

 dies and decays. In plants all is growth, as distinguished from the 

 reproduction of removed or decayed parts, which is the office of the 

 nutrition of animals.* 



" Absorption. — From what has already been said with regard to 

 peculiarities of cell-membrane producing the phenomena of endos- 

 mose, it will readily be seen how perfectly the structure of the fibrillae 

 of the roots is adapted to the absorption of the fluids around them. 

 The nature of the development, too, of the radical tissues, the root 

 always growing by its extremity, continually furnishes fresh cells in 

 the most favourable condition for absorption. The absence of epi- 

 dermis, that denser and more impermeable layer of tissue which is 

 produced in other parts to moderate the transmission of fluids, is 

 another important condition in the absorbing extremities ; the deli- 

 cate epihlema by which young roots are clothed being composed of 

 cells which have lost none of their absorbent power, while their ap- 

 position as a continuous layer guards against the entrance of solid 

 matter into the cavities of the internal parenchyma. The roots ab- 

 sorb only fluid, and all substances which afterwards present them- 

 selves in a solid form within the cells, such as crystals, &c., must 

 have entered the plant in a state of solution. Experiments have been 

 made, placing the roots of plants in water containing finely powdered 

 solids, such as charcoal and colouring matters ; these were always 

 found to accumulate upon the surface of the root, but never to pene- 

 trate the tissues. 



" Although roots thus reject all solia matter (and this is most pro- 

 bably a simple mechanical necessity), they do not appear to have 

 any power of selection ; they absorb poisons as readily as innocuous 

 or beneficial fluids. Difference of the relative densities of fluids, as 

 would follow from the recognition of endosmosis as the agent of ab- 

 sorption, is the only circumstance which requires any manifestation 

 of preference by the absorbing surfaces." — p. ^Q. 



* Of course this generalization does not apply to ultimate or elementary tissues, but 

 to tbose assemblages of struct nves for a special function commonly known as organs. 



