MICROSCOPIC FORMS OF VEGETABLE LIFE. 225 



OITS modifications to be described in their proper place (Chap. vin.). 

 A composite structure is thus developed, which is made up of a number 

 of distinct ' organs' (stem, leaves, roots, flowers, etc.); each of them 

 characterized by specialties not merely of external form, but of intimate 

 structure; and each performing actions peculiar to itself, which contri- 

 bute to the life of the Plant as a whole. Hence, as was first definitely 

 stated by Schleiden, it is in the life histo ry of the individual cell that we 

 find the true basis of the study of Vegetable Life in general. 



223. 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 consists of two layers, differing entirely in composition and 

 properties. It is the inner, termed the ' primordial utricle/ that is first 

 formed, and is most essential to the existence of the cell; it is extremely 

 thin and delicate, so that it escapes attention so long as it remains in con- 

 tact with the external layer; and it is only brought into view when sep- 

 arated from this, either by developmental changes (Fig. 141, A), or by 

 the influence of reagents which cause it to contract by drawing-forth 

 part of its contents (Fig. 139, c). It is not sharply defined on its inter- 

 nal face, but passes gradationally into the protoplasmic substance it in- 

 closes, from which it is chiefly distinguishable by the absence of granules. 

 And it is shown by the effects of re-agents to have the albuminous com- 

 position of protoplasm. It may thus be regarded as the slightly con- 

 densed external film of the protoplasmic. layer with which its inner sur- 

 face is in contact; and as it essentially corresponds with the 'ectosarc' 

 of Amoeba or any other Rhizopod ( 396), it may be termed the ecto- 

 plasm. The outer layer, on the other hand, entirely consists of cellulose, 

 which seems to be excreted from the surface of the * ectoplasm ' for the 

 protection of its contents; it is usually thick and strong, and can often 

 be seen to consist of several layers. The ' ectoplasm ' and ' cellulose wall ' 

 can be readily distinguished from one another by Chemical tests ( 204); 

 and also by the action of Carmine, which stains the protoplasmic sub- 

 stance (when dead) without affecting the cellulose-wall. 



224. The contents of the Plant-cell, which may be collectively termed 

 the endoplasm (answering to he 'endosarc' of Rhizopods), or, when 

 strongly colored throughout (as in many Algae) the endochrome, consists 

 in the first place of the layer of protoplasmic substance which lines the 

 ' ectoplasm;' secondly, of a watery fluid, called ' cell-sap,' which holds 

 in solution sugar, vegetable acids, saline matters, etc. ; thirdly, of the pe- 

 culiar body termed the 'nucleus;' and fourthly, of chlorophyll-cor- 

 puscles (inclosing starch-granules), oil-particles, etc. In the young state 

 of the cell, the whole cavity is occupied by the protoplasmic substance, 

 which is, however, viscid and granular near the cell -wall, but more watery 

 towards the interior. With the enlargement of the cell and the imbibi- 

 tion of water, clear spaces termed vacuoles, filled with watery cell-sap, 

 are seen in the protoplasmic substance ; and these progressively increase 

 in size and number, until they come to occupy a considerable proportion 

 of the cavity, the protoplasm stretching across it as an irregular network 

 of bands. Where, as usually happens, the ' nucleus ' lies imbedded in 

 the outer protoplasmic layers, these bands are gradually withdrawn into 

 it, so that the separate vacuoles unite into one large general vacuole which 

 is filled with watery cell-sap. But where the ' nucleus ' occupies the cen- 

 tre of the cell, part of the protoplasm collects around it, and bands or 

 threads of protoplasm stretch thence to various parts of the parietal layer. 

 It is by the contractility of the protoplasmic layer, that the curious ' cv- 



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