ELEMENTS OF ORGANIC STKUCTUEE. 



231 



cases, however, the cells of plants are so small as to re- 

 quire a i^owerfiil microscope to distinguish them, — arc, in 

 fact, no more than l-1200th to l-200th of an inch in diam- 

 eter ; many are vastly smaller. 



Growth. — The growth of a plant is nothing more than 

 the aggregate result of the enlargement and multiplication 

 of the cells which compose it. In most cases the cells at- 

 tain their full size in a short time. The continuous growth 

 of plants depends, then, chiefly on the constant and rapid 

 formation of new cells. 



Ccll-inultiplication. — The young and active cell always 

 contains a nucleus, (fig. 34, h.) Such a cell may produce 

 a new cell by division. In this process 

 the nucleus, from which all cell-growth 

 appears to originate, is observed to re- 

 solve itself into two f)arts, then the 

 protoplasm, «, begins to contract or in- 

 fold across the cell in a line correspond- 

 ing with the division of the nucleus, until 

 the opposite infolded edges meet — like 

 the skin of a sausago where a string is 

 tightly tied around it, — thus separating the two nuclei and 

 inclosing each within its new cell, which is comiDleted by 

 a further external growth of cellulose. 



In one-celled plants, like yeast, (fig. 35,) the new cells 

 thus formed, bud out from the side 

 of the parent-cell, and before they 

 obtain full size become entirely 

 detached from it, or, as in higher 

 plants, the new cells remain adher- 

 ing to the old, forming a tissue. 



In free cell-formation nuclei are observed to develope 

 in the protoplasm of a parent cell, which enlarge, surround 

 themselves with their own protoplasm and cell-membrane, 

 and by the resorption or death of the parent cell become 

 independent of the latter. 



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