STRUCTURE OF PLANTS. 47 



itself only in the form of separate rings, (PL i, Fig. 7.) 

 These are named, according to these appearances of the 

 thickening layers of the cell, porous or streaked cells, 

 reticulated, spiral, or annular fibrous cells. When one 

 thickening layer has been formed in this manner, a second 

 and third frequently follow, and often so on until almost 

 the whole cavity of the cell is filled up. It will easily be 

 conceived how from these changes, in conjunction with 

 the variations of form mentioned just before, even from 

 so simple an element as the cell may proceed an almost 

 countless number of differences in the tissue, and in plants 

 we actually find such. Added to this, foreign matters, such 

 as lime, silex, &c., are frequently deposited in the cell-wall 

 and its thickening layers, whence originate numerous 

 modifications of softness and hardness, tenacity and 

 brittleness. 



We have yet one more important peculiarity of the 

 vegetable cell to discuss before we can go further. If the 

 nutrient matter, within the cell, increases in quantity beyond 

 a certain measure, new cells are formed from it within the 

 first, called secondary or daughter-cells ; they propagate, 

 and in the usual course the mother-cell then gradually 

 dissolves and disappears, while the two, four, eight or more 

 young cells produced by it, occupy its place. The whole 

 process, which we call growth, in plants, consists in its 

 essential elements of a continuous propagation of cells of 

 this kind, whence the number of cells becomes multiplied 

 beyond calculation, nay, almost beyond credibility. From 

 an approximative calculation, for example, in a rapidly 

 growing fungus, the Bovista gig 'ant ea, 20,000 new cells 

 are formed every minute. 



Elegant as the above-described forms of the cells may 

 be when seen beneath the microscope, interesting as 



