VEGETABLE CELLS IN GENERAL. 241 



of tlieir lives) possess a power of spontaneous movement, but also 

 that the instruments of motion (when these can be discovered) are 

 of the very same character in the Plant as in the Animal ; being 

 little hair-like filaments termed Cilia (from the Latin cilium, an 

 eye-lash), by whose rhythmical vibration the body of which they 

 form part is propelled in definite directions. The peculiar contrac- 

 tility of these Cilia cannot be accounted for in either case, any 

 better than in the other ; all we can say is, that it seems to depend 

 upon the continued vital activity of the living substance of which 

 these filaments are prolongations, and that this contractile sub- 

 stance has a composition essentially the same in the Plant as in the 

 Animal. 



181. The plan of organization throughout the Vegetable kingdom 

 presents this remarkable feature of uniformity, — that the fabric of 

 the highest and most complicated Plants consists of nothing else 

 than an aggregation of the bodies termed Cells, every one of which, 

 among the lowest and simplest forms of Vegetation, may maintain 

 an independent existence, and may multiply itself almost indefin- 

 itely, so as 'to form vast assemblages of similar bodies. And the 

 essential difference between the plans of structure in the two cases 

 lies in this : — that the Cells produced by the self-multiplication of 

 the primordial cell of the Protophyte are all mere repetitions of it 

 and of one another, each living by and for itself, — whilst those 

 produced by the like self-multiplication of the primordial cell in 

 the Oak or Palm not only remain in mutual connection, but undergo 

 a progressive 'differentiation ;' a composite fabric being thereby 

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

 Leaves, Roots, Flowers, &c), each of them characterized by speci- 

 alities not merely of external form but of intimate structure (the 

 ordinary type of the Cell undergoing various modifications, to be 

 described in their proper place, Chap, vih.), and each performing 

 actions peculiar to itself which contribute to the life of the Plant 

 as a whole. 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. And we shall now 

 inquire, therefore, what information on this point we derive from 

 Microscopic research. 



182. In its most completely-developed form, the Vegetable Cell 

 may be considered as a closed membranous bag or vesicle, contain- 

 ing a fluid cell-sap ; and thus we have to consider separately the 

 Cell-icall and the Cell- contents. The Cell-wall is composed of 

 two layers, of very different composition and properties. The inner 

 of these, which has received the name of Primordial Utricle, ap- 

 pears to be the one first formed and 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 contact with the external layer ; 

 and it is only brought into view when separated from this, either 

 by developmental changes (Fig. 153), or by the influence of re- 



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