DEVELOPMENT AND METAMORPHOSES OF CELLS. 109 



these appear to have been formed by the breaking-down of the transverse 

 partitions, between a regular series of cylindrical cells laid end to end ; and 

 the remains of such partitions may frequently be seen in them. The ducts 

 which convey the ascending sap, do not inosculate with each other ; their 

 purpose being merely to carry it direct to the leaves ; but the vessels, through 

 which the descending or elaborated sap flows, are of very different character; 

 for their purpose is to distribute the nutritious fluid through the tissues ; 

 and they anastomose very freely, just as do the capillaries of Animals. The 

 network which they form, however, can be as clearly traced to an origin in cells, 

 whose cavities were originally distinct, as can the bundles of straight non- 

 communicating ducts. Another important transformation of the original cells, 

 is that by which the Woody Fibres, which compose nearly all the fibrous 

 textures of Vegetables, are produced. These fibres are still cells, but their form is 

 very much elongated ; 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. 



121. Such deposits may take place in cells of the ordinary form ; and they 

 present many variations in their character, which give corresponding peculi- 

 arities to the cells which contain them. In many instances, they consist merely of 

 concentric layers, one within the other, each layer completely lining the one 

 which preceded it; and the cavity of the cells being thus gradually but uni- 

 formly contracted in every dimension. In other cases, certain points of the 

 original external cell-membrane are left uncovered by the secondary deposits ; 

 and thus, the same vacuities being left in the successive layers, passages are 

 formed, which stretch out from the central cavity to certain spots of the peri- 

 phery of the cell. Cells of this character are found in certain parts of plants, 

 which are required to possess unusual firmness, without losing the power of 

 transmitting fluid, the former endowment being conferred by the secondary 

 deposits; whilst the latter is retained by the peculiar system of passages just 

 described, the thin or uncovered parts of the wall of one cell being in contact 

 with corresponding spots on the walls of adjacent cells, as we see in the tissue 

 of the stones of fruit, the central gritty matter of the pear, &c. Lastly, the 

 new deposit may present the form of a more or less regular spiral fibre, winding 

 within the cell from end to end ; and this may present itself alike in cells of 

 the ordinary shapes, or in fusiform cells (constituting the proper spiral ves- 

 sels], or in cells that have coalesced into continuous tubes or ducts. The 

 spiral may break up into rings or irregular pieces ; and these may be united 

 again by additional deposits of a still more irregular character, so as completely 

 to obscure theiroriginal spiral form. This spiral fibre is very completely gene- 

 rated, in some instances, when the cell-wall itself has not acquired any greater 

 tenacity than that of mucus, very easily dissolved ; which (as we shall presently 

 see) is a stage in the production of cells in general. Such spiral fibres spring 

 out from the external coats of many seeds, when they are moistened with 

 fluids. 



122. So far as is yet known, all Cells originate in germs, that have been pre- 

 pared by some previously-existing cell ; and these germs may either be de- 

 veloped within the parent-cell, or may be set free by its rupture, and may be 

 developed quite independently. The latter case, being the simplest, will be 

 first considered ; we have numerous examples of it among the lower Cellular 

 Plants. In the first place, the germ, from which the cell originates, is a mi- 

 nute granule, only to be seen with a good microscope, and apparently quite 

 homogeneous. It has the power of drawing to itself the nutrient elements 

 around, and of combining these into the proximate principles, that may serve 

 as the materials for its development. By the incorporation of these with its 

 own substance, it gradually increases in size, and a distinction becomes ap- 



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