378 Vegetable Physiology. 



air and liglit to insure thoroug-h assimilation, the formative con- 

 tents of the older cells are carried away to supply material for 

 new growth, or become dihited as it were in the unnaturally- 

 expanded cells, and the result is the production of a weak, watery 

 mass of tissue. The more minute consideration of these points 

 must.be reserved until we come to speak of the cell-contents. 



We have said that in the succulent tissues the cells become 

 little altered except in form and dimensions ; but even in the 

 most delicate of these tissues the membranous ' cell-wall ' ac- 

 quires a certain increase of solidity beyond its original condition. 

 Tlie degree of expansion of each cell of any tissue, being con- 

 fined within certain limits, the nutrient agency of the formative 

 layer lying upon the internal surface is subsequently directed 

 to the increase of thickness of the membrane, which is effected 

 by the application of new, delicate lamellae upon the inner 

 surface of the primary membrane, intimately adhering to it. In 

 ordinary cellular tissues these layers of thickening are thin and 

 few in number, and could hardl}- be detected were it not that 

 they are to a certain extent incomplete, leaving certain spots of 

 various forms of the primary meml)rane uncovered, which thinner 

 places appear like holes or slits in the cell-wall, although this 

 is not actually perforated. These thinner places occur chiefly on 

 parts of the cell-wall adjacent to other cells, and appear to con- 

 stitute a means of facilitating the passage of fluids from the 

 cavity of a cell into those of its neigti hours. In certain cases 

 the laj-ers of thickening are more numerous, giving a far more 

 solid character to the tissue, which still retains its succulent 

 character ; tliis is especially observed in the cells of fleshy 

 leaves, in the parenchyma of the rind, or of the pith of annual 

 shoots of particular plants, &c. 



From cells of this character it is but a step to those which are 

 met with occasionally in organs of transitory duration, which 

 retain the parenchymatous form and active vital functions, while 

 their walls become greatly altered in character. This is the case 

 in certain organs where the cellular tissue is organized as a reser- 

 voir of accumulated nutriment, to preserve this during a season 

 of rest, l)ut where this nutriment is laid up, not in the cell- 

 contents, but in great part in structures belonging to tlie cell-wall. 

 The starch of the wheat-grain, or of the potato tuber, is laid up 

 in delicate membranous sacs, cells which preserve all their 

 original characters as regfards the condition of the ' cell-wall.' 

 But the seed-lobes of the bean and pea, the ' endosperm ' of the 

 onion seed and of many other plants, are composed of tissue in 

 which the cells have their walls so greatly thickened that their 

 cavities are comparatively small, and the coherent mass of cells 

 acquire a fleshy or horny texture (fig. 2). It may be mentioned 



