CELL. 



[ 118 ] 



CELL. 



word cell is not distinctive enough, and will 

 beget confusion; hence it seems desirable to 

 apply a special name to the newly-detected 

 and definite form of organization, the free 

 protoplasmic globule. The term primordial 

 utricle seems to answer all requirements, 

 since the isolated body is chemically and 

 physiologically identical with the ordinary 

 primordial utricle lining a nascent cell, having 

 in like manner the function of forming a true 

 cell by secreting a layer of cellulose all over 

 its external surface and thus enclosing itself 

 in a sac. 



In this work, then, the word cell, as applied 

 to vegetable structures, is always used in its 

 ordinary sense, and the character of the 

 "primordial-cell" of Cohn and other German 

 authors is given under the head of Primor- 

 dial Utricle. 



We have already defined a cell, above ; we 

 next proceed to describe the form and size. 



Form. — Cells may present almost every 

 possible modification of form, and this 

 depends on two sets of conditions, the ori- 

 ginal development and shape, and the mode 

 of growth and expansion. It is frequently 

 stated that the primary form of all vegetable 

 cells is that of a sphere, or at all events that 

 this is the type from which all the others 

 must be considered deviations. This is true 

 only so far as it is intended to signify, that 

 all cells which originate free in the midst of 

 fluid, suffering no external compression, 

 have a globular form, and that in numerous 

 cases where cellular tissues are very lax and 

 free to expand in all directions, the compo- 

 nent cells acquire a globular form during 

 the enlargement to their full size. But in a 

 very large majority of cases the cells do not 

 originate in a free condition, they are pro- 

 duced by subdivision of older cells, and 

 consequently, when first develo])ed, they have 

 the shape of the half, the quarter, or what- 

 ever segment it may be of the parent-cell ; 

 moreover, in a majority of these cases the 

 mode of expansion also depends upon a 

 special law of the particular tissue, or even 

 of such tissue in the particular group to 

 which the plant belongs, and not upon any 

 general law of globular expansion. This 

 law does prevail widely in some families, as 

 in the Fungi, and we very frequently see it 

 prevailing in pith up to a certain period, but it 

 will not hold as a general rule, for the lax 

 tissues of leaves, of succulent stems, &c., otFer 

 most striking deviations, as do also the 

 cylindrical tubular forms so widely ])revalent 

 in the lower Algge. It is further stated in 



many books, that the effect of pressure on 

 cells having a tendency to become globular, 

 is the production of a dodecahedral form, but 

 this again is far too sweeping a generaliza- 

 tion, and the real fact is, that globular cells 

 of equal size expanding in a confined space, 

 often become twelve -sided by mutual pres- 

 sure, but far more often the cells of a tissue 

 are of diverse size, and hence a polyhedral 

 form is much more common (fig. 115). Cells 



Fig. 115. 



may be globular, as in the Yeast-plant, and 

 many lower Algae, in the lax tissue of young 

 pith of many Dicotyledons (PI. 38, fig. 14), 

 &c. ; oval, as is much more common in 

 parenchymatous tissues ; squarish, as in 

 cork (PI. 38. figs. 16, XT); or tabular, as 

 in the epidermis of numerous plants, under 

 which circumstances the side- walls may be 

 square, rhombic (H3^acinth-leaf), hexagonal or 

 irregular, as in many petals ; and the out- 

 lines may also be undulated or even beauti- 

 fully zig-zagged, as in the leaf of Helleborus 

 fcbtidus, &c., the petals of many flowers, or 

 in the leaf of the Pine-apple (PL 38. fig. 15), 

 &c. ; while the upper exposed face may be 

 flat or vaulted, as in most petals, or even 

 papilliform, as on the petals of the Sweet- 

 William, and of most flowers 

 with glistening surface. Cells 

 may alsobe cylindrical,andthen 

 either with flat ends (fig. 116), 

 as in the parenchyma of many 

 Monocotyledons and in the fila- 

 ments of Confervse, or round- 

 ed ends, or attenuated ends, as 

 in wood and liber tissue gene- 

 rally ; or they may be pris- 

 matic, and then square, or six- 

 sided, as in stems of most 

 herbaceous plants ; spindle- 

 sha})ed, as in a large number of woods, such 

 as that of Conifers, Box, &c., and, in fact, 

 of almost every conceivable form. In lax 

 tissues, the walls of the cells often grow very 

 unequally at different points, whence result 

 angular j)rojections (by which the cells ordi- 

 narily cohere together) (fig. 117); or these 



Fig. 



116. 



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