FORMATION OF CELLS. 419 



Woody Fibres, which compose nearly all the fibrous textures of Vegetables, 

 are produced. These fibres are still cells, but their form is very much elon- 

 gated ; they have a fusiform or spindle shape, being tubes drawn to a point at 

 each end; at first they are quite pervious, like ordinary cells; but in the older 

 wood, their cavity is filled up by interior deposit. There seerns reason to 

 believe, that fasciculi of these fibre-cells originate within certain of the ordi- 

 nary cells of the primary cellular tissue ; for in the young Plant, the latter 

 alone can be detected ; and it is not until the operation of the leaves has fairly 

 commenced, that any true woody structure is formed. Thus, cells or vesicles 

 may be regarded as the primordia of all the Vegetable tissues. The next 

 question is, how are Cells formed ? 



657. Cells appear to originate in two modes ; either in the midst of an 

 organizable fluid, under the influence of a living solid tissue with which it is in 

 contact, or in the interior of previously-formed cells. Both these modes may 

 be observed in the Animal as well as in the Vegetable organism ; and the right 

 comprehension of them is of the utmost importance. It has been already 

 remarked, that Gum holds the same rank in the economy of the Plant as 

 Albumen does in that of the Animal ; and the glutinous compound which exists 

 in the elaborated sap, and which is especially abundant in parts where organ- 

 ization is taking place with rapidity, may be compared with fibrin. This glu- 

 tinous sap undergoes a sort of coagulation when withdrawn from the vessels, 

 and it may be sometimes perceived to resolve itself into distinct organic forms. 

 The process of organization may be observed with the greatest facility in the 

 embryonal sac, previously to fecundation. This contains, when first developed, 

 a consistent gummy fluid, slightly wanting in transparency, but not exhibiting 

 any distinguishable granules ; the addition of tincture of iodine produces a sort 

 of granular coagulum, of a pale yellow. The first perceptible stage of organ- 

 ization is the appearance in this fluid of a number of extremely minute granules, 

 which render it opalescent and almost opaque. The fluid then takes from 

 iodine a somewhat darker tinge, and the granules, when, their small size per- 

 mits their colour to be distinguished, seem to become of a dark brownish-yellow. 

 Single, larger, and more sharply-defined granules are next evident in the mass, 

 and these soon present a regular form, and increase in size 1 apparently from 

 the coagulation of the minuter granules around the larger ones. These bodies 

 usually assume a flattened disc-like form, with a circular or oval outline ; and 

 as they speedily become subservient to the formation of cells, they have been 

 termed cytoblasts or cell-germs. From the surface of each of these, a delicate 

 transparent membrane is seen to project, as a watch-glass does from the dial ; 

 and this is the commencement of the cell. The membrane gradually projects 

 more and more, and extends beyond the cytoblast, which is at last seen as a 

 mere spot upon its walls. It is some time in acquiring consistence ; for even 

 after the cell has arrived at nearly its full size, it may be made to dissolve by 

 agitation in the surrounding fluid. In fact, a disappearance not unfrequently 

 takes place, as a part of the natural course of vital phenomena ; the cell-walls 

 melting away before they have acquired consistence enough to be permanent. 

 When the cell is complete, the granular cytoblast commonly disappears ; some- 

 times, however, it remains in the wall of the cell, where (in the orders Orchideas 

 and Cactese) it was long since described by Dr. R. Brown, under the name of 

 the nucleus. By Schleiden, the original observer of these phenomena,* it is 

 considered that the function of the cytoblast is complete with the formation 

 of the primordial cell ; but there is strong reason to believe, that the granules 

 of which it is composed are the germs of new cells afterwards to be developed 



* See Muller's Archiv., 1808, p. 137, Taylor's Scientific Memoirs, vol. ii., and the Brit. 

 and For. Med. Rev., vol. ix. p. 499. 



