THE VEGETABLE CELL 533 



casing of the substance termed cellulose a non-nitrogenous substance 

 identical in chemical composition with starch. The entire mass of 

 cells of which any vegetable organism is composed has been gene- 

 rated from one ancestral cell by processes of multiplication to be 

 presently described ; and the difference between the fabrics of the 

 lowest and of the highest plants essentially consists in this, that whilst 

 the cells produced by the repeated multiplication of the ancestral 

 cell of the protophyte are all mere repetitions of it arid of one an- 

 other each living by said for itself, those produced by the like multi- 

 plication of the ancestral cell in the oak or palm not only remain in 

 mutual connection, but go through a progressive ' differentiation,' 

 the ordinary type of the cell undergoing various modifications to be 

 described in their proper place. A composite structure is thus 

 developed, which is made up of a number of distinct ' organs ' (stem, 

 leaves, roots, flowers, <fec.), each of them characterised by specialities 

 not merely of external form, but of internal structure ; and each 

 performing actions peculiar to itself, which contribute to the life 

 of the plant as a ivhole. Hence, as was first definitely stated by 

 Schleiden, it is in the life-history of the individual cell that we 

 find the true basis of the study of vegetable life in general. 



We have now to consider in more detail the structure and life- 

 history of the typical plant-cell, and shall begin by treating of the 

 cell-wall. This cell-wall is composed, as long as the cell is in a 

 living state, chiefly of the substance known as cellulose, one of the 

 group of compounds called * carbohydrates,' and bearing the definite 

 chemical composition C G H 10 O 5 . From a physical point of view it 

 consists of particles or micellce of cellulose surrounded by water. 

 In addition to cellulose, recent observations have shown that pectic 

 substances enter largely into the composition of the wall of the 

 living cell, especially in its early stages. In fungi it is doubtful 

 whether there is any true cellulose in the cell-walls. With regard 

 to the mode of growth of the cell-wall, two hypotheses have been 

 proposed : one, that it is formed by apposition, that is, by the 

 constant addition of fresh layers to the inner surface of the cell-wall ; 

 the other that it increases by intussusception, or the intercalation of 

 fresh particles of cellulose between those already in existence. The 

 results of modern researches tend in the direction of the former 

 being the more usual process ; but it is probable that the two co- 

 operate in producing the total growth of the cell-wall. 



The contents of the plant-cell, which may be collectively termed 

 the endoplasm (answering to the * endosarc ' of rhizopods), or, when 

 strongly coloured throughout (as in many algae), the endochrome, 

 consist in the first place of an outer layer of protoplasmic substance 

 called the ectoplasm , primordial utricle, or parietal utricle. This is an 

 extremely thin and delicate layer, so that it escapes attention so long 

 as it remains in contact with the cell-wall ; and it is only brought 

 into view when separated from this, either by developmental changes 

 (fig. 415), or by the influence of reagents which cause it to con- 

 tract by drawing forth part of its contents (fig. 413, C). It is not 

 sharply defined on its internal face, but passes gradually into the 

 inner mass of protoplasm, from which it is chiefly distinguishable by 



