128 



PART II. —VEGETABLE HISTOLOGY. 



the latter imbibes more water than it is capable of holding in 

 solution, and so sap cavities or vacuoles are formed in it, which, 

 at the maturity of the cell, often occupy more space than the 

 protoplasm itself, and, finally, 

 when the cell grows old, its 

 living contents disappear, and 

 the cell becomes dead matter. 

 The term cell, as applied to 

 the structural unit of plant life, 

 is not strictly appropriate, since 

 it contains the idea of an in- 

 closure. When first used in this 

 sense it was because of a mis- 

 taken notion regarding its true FlG . 362 _ Young cells from near the apex of a 



Character; it Was Supposed tO growing root. Magnified about 300 diameters. 



be a closed cavity, and the subordinate nature of the cell-wall 

 was not understood ; but we now know that the protoplasm, and 

 not the cell-wall which incloses it, is the essential part, and that 

 an uninclosed, nucleated mass of protoplasm, that is in no sense 

 a closed cavity, is capable of performing all the essential func- 

 tions of a cell, and must be regarded as such. It is, however, 

 better to retain the old word and give to it an extended meaning 

 than to discard a term which long usage has rendered familiar. 



Cells which are destitute of a cell-wall are called primordial 

 or naked cells. Though common in the animal kingdom, they 

 are comparatively rare in the vegetable world, occurring mostly 

 among the low forms of plant life. 



Let us now consider more particularly the different parts ot 

 the cell. 



The Cell-Wall. The cellulose of the cell-wall is not in 

 the proper sense living matter, although when young it is perme- 

 ated by protoplasm which is living matter. As the cell grows 

 older, the protoplasm contained in its wall diminishes in quan- 

 tity, and finally disappears, while the cellulose increases in 

 quantity. These, among other facts, lead to the conclusion that 

 it is produced by the transformation of the protoplasm. At its 

 first formation the cell-wall is a delicate film of even thickness, 

 but as growth proceeds, it not only becomes thickened to a 

 greater or less extent, but usually unequally so, and this gives rise 



