36 THE ELEMENTARY STRUCTURE OF PLANTS. 



ing thinner as it expands, it grows thicker ; although the increase 

 of surface at this time is much greater than that of thickness. 

 Therefore it not merely enlarges, but grows. That is, it incorpo- 

 rates new assimilated matter, which penetrates the membrane and 

 is deposited in it, not as a new layer, lining and strengthening the 

 old, but inter stitially ; so that the enlarging cell-wall is still as 

 homogeneous and simple as before. After attaining, for the most 

 part rapidly, a definite size, the cell ceases to enlarge, and its wall 

 no longer incorporates new materials. Some cells remain in this 

 condition, with walls of great tenuity, as do the parent cells in 

 which grains of pollen or other new cells are produced (31) ; in 

 which case they seldom endure, but are soon destroyed or ab- 

 sorbed. The assimilated matters they contained were wholly 

 diverted to the new product to which they give rise. 



39. Thickening by Deposition, In most cells that make part of a 

 permanent structure, however, the membrane continues to thicken 

 after it has ceased, or nearly ceased, to enlarge, no longer inter- 

 stitially, but by a deposit on its inner surface. The nature of the 

 contained assimilated matter is such, that, by the mere abstraction 

 of water, it readily passes into a solid state (81). As it organizes 

 (doubtless under the influence of the living lining of protoplasm), 

 it solidifies on the surrounding cell-wall, which is thus strengthen- 

 ed by a new layer of cellulose, or by a succession of such layers. 

 Every degree of this secondary deposition occurs, from a slight 

 increase in the thickness of the membrane to the filling up of the 

 greater part of the cavity of the cell. The older wood-cells of any 

 hard wood furnish good illustrations of such solidification. Indeed, 

 the difference between sap-wood and heart-wood of trees is princi- 

 pally owing to the increase of this secondary deposit, which con- 

 verts the former into the latter ; as may be seen by comparing, 

 under the microscope, the tissue of the older with that of the newest 

 rings of wood, taken from the same tree. In an ensuing chapter 

 (on the internal structure of the stem), this is shown in a piece of 

 oak wood. Fig. 18 represents a highly magnified cross-section of 

 some wood-cells from the bark of a Birch, with their calibre almost 

 obliterated in this way. It is by the same process that the tissue 

 of the stone of the peach, cherry, and other stone-fruits acquires its 

 extreme hardness. Indurated cells of the same kind are met with 

 even in the pulp of some fruits, as in the gritty grains, which every 

 one has noticed, scattered through the flesh of many pears, espe- 



