248 MICROSCOPIC FORMS OF VEGETABLE LIFE. 



other of their lives) possess a power of spontaneous movement, 

 but also that the instruments of motion, when these can be dis- 

 covered, 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 eyelash), by whose rhythmical vibration the 

 body of which they form part is propelled in definite directions. 

 The peculiar contractility 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 substance has a composition essentially the 

 same in the Plant as in the Animal. 



147. 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, amongst the lowest and simplest forms of 

 Vegetation, may maintain an independent existence, and may 

 multiply itself most indefinitely, 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 fabric being thereby developed, which is 

 composed of a number of distinct organs (stem, leaves, roots, 

 flowers, &c.), each of them characterized by specialities 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. VIII), and performing actions pe- 

 culiar to itself, which contribute to the life of the Plant as a whole. 

 Hence, as was first definitely stated by Schleiden (see Introduc- 

 tion, p. 43), 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. In its most com- 

 pletely developed form, the Vegetable Cell may be considered as 

 a closed membranous bag or vesicle, containing a fluid cell-sap ; 

 and thus we have to consider separately the cell-wall 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, appears 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 develop- 

 mental changes (Fig. 107), or by the influence of reagents which 



