THE VEGETABLE CELt;. 63 



A. THE CELL AS AN ORGAN OF NUTEITION. 



A. ABSORPTIOlsr OF WATERY FLUIDS. 



In all plants tlie fluid nutriment is taken up by absorption 

 through cells. As the cell-membrane has no orifices, only such 

 matters as are actually dissolved, can be absorbed into the cell with 

 the water which penetrates the cell-membrane ; in like manner, 

 in all the higher plants, a mechanical penetration of solid sub- 

 stances, even when suspended in water in the finest state of divi- 

 sion, between the cells into the interior of the plant, is impossible, 

 since the cells which form the surface of the plant are accurately 

 fitted together, leaving no orifices between, except the stomates, 

 which never occur upon roots or parts growing under water. 



Gaseous fluids, by which the cell- walls are also readily penetrable, 

 may in like manner be taken up by the cells situated at the sur- 

 face; but they can moreover penetrate between the cells, into the 

 interior of plants, through the stoniates. 



Ohserv. The Thallophytes possess no proper organ of absorption, but 

 the -whole of their surface is adapted for it, and when, as in many Algae and 

 Lichens, they have root-like processes, these are only organs of attachment 

 and not special organs of absorption. In many Fimgi and Lichens the sub- 

 stance of the tliallus is composed of sneh loosely connected cells, that fluids 

 which come in contact with them penetrate between the cells into the sub- 

 stance of the thallus, so that the absorption is not confined to the superfi- 

 cial cells here. Eyen in the Mosses the root does not make any considerable 

 figure as an absorbing organ, their freely penetrable leaves being the chief 

 agents of the absorptirn of water. In the liigher plants absorption is com- 

 mitted to the root alone, since the epidermis of the leaves and the periderm 

 of the other parts are much too difficult of penetration by water, to be 

 capable of taking up a sufficient quantity of it. This obstruction occurs 

 even in the root except at the yoimg parts situated near the points. Con- 

 sequently, if a plant be placed in water in such a manner that its younger 

 roots are curved up above the surface, it dries up, while it keeps fresh 

 when only the younger roots (not however the extreme points, known by 

 the name of spongioles, alone) are immersed in water. The parts, how- 

 ever, the leaves particularly, protected against the entrance of fluid water, 

 are readily penetrable by watery vapour, and plants can in this way appro- 

 priate water from very moist air, as is clear from the increase of weight of 

 entire plants or cut twigs j this explains the great use of dew to the vege- 

 tation of dry, hot regions. 



It has long been decided, that solid substances, insoluble in water, no 

 matter how finely they are powdered, e. </., the charcoal of gunpowder, can- 

 not pass into plants ; but this may be doubtful of the colouring matter of 

 Phytolacca, of decoction of log- wood, of infiision of saffron, &c., since many 

 observers, e, ^., De Candolle, have seen such colouring matters pass into 

 living plants. But all accurate observations indicate that this does not 

 happen in uninjured roots, but only occurs when the coloured fluid comes 

 in contact with wounds of the plants. 



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