10 



CELLULAR TISSUE. 



l-'i-. 22. Cells from the Straw- 

 berry, showing their oval 

 shape, loose connexion, large 

 nucleus, and translucent 

 walls. 



and add a drop of water. Pull it to pieces by the help of two needles, and then cover 

 it with thin glass, and place it under the microscope. It 

 will be found to consist of a mass of large cells (Fig. 22), 

 with transparent walls, and a slightly coloured fluid, in- 

 closing the large rounded nucleus. It is of importance 

 to'obtain clear notions of a cell, since it is the foundation 

 of all other tissues, and since it contains the starch and 

 all other secretions of plants. 



The figure of the cell is unimportant, and varies very 

 greatly. It is believed to be generally accidental, as the 

 phrase is, the accident being that of pressure : not that by 

 the term " accident" is meant that the figure is a matter of 

 chance ; for in certain parts of plants, as in the pith, for 

 example, the figure, whatever it may be, is always the 

 same. If pressure, therefore, in such cases be the efficient 

 cause, it is exerted in determinate degrees and directions in the various parts of plants. 

 \Vhen the schoolboy blows bubbles of soap-and-water he makes rounded cells, because 

 the walls are of equal weight, and the pressure of the air of an even degree all round. 

 If, however, a drop of water be attached to the bubble it will destroy its rounded 

 form, and elongate it in the direction of the earth, rendering the ceil more or less oval. 

 But if the same soap-and-water be well shaken in a half-filled bottle, the unequal 

 pressure will drive the cells together, and render them distinctly six-sided. 



This little experiment will convince the reader that the figure of the cell does, in a 

 great degree, depend upon pressure, and that it may be altered as the direction or 

 degree of pressure is changed. 



So also in plants when each cell is detached from every other, as in decomposing 

 vegetable infusions ; or aa in the yeast plant (Torula Cerevuia-Fig. 16), the form is 

 spherical or ovoid ; when it lies loosely in juicy fruits, as in the strawberry (Fragaria 

 Fig. 22), it is large and nearly round ; when two or more cells are attached end to end, 

 as in the mushroom (Fig. 23), they are ovoid or elongated; 

 and when they are numerous and inclosed in a common skin 

 or bark, they become more or less six-sided, as in the pulp 

 of the orange (Citrus), from mutual and surrounding pressure 

 (Fig. 24). It will then be readily understood that the figures of 

 cells may be innumerable ; but experience has shown that hexa- 

 gonal and octagonal forms are those which most abound. These 

 are the forms observed almost uni- 

 versally in pith, cuticle, leaves, flow- 

 ers, and fruit ; but it should be re- mush attached end 

 membered that regularity of outline, to end. 

 although of common occurrence, is by no means'essential. 



But, whilst it must be admitted that the figure, in most 

 instances, results from pressure, in other instances it pro- 

 ceeds from a more determinate source ; viz., the direction 

 of the growing process. This is readily understood, if we 

 imagine a spherical cell in which the growing process 

 is not equally carried on all over it, so that it may 



'ir\ 



Fig. 24. Hexagonal cells. 



Continue to grow spherical ; but whilst the process is arrested at one point it proceeds 



