ELEMENTS OF ORGANIC STRUCTURE. 231 



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

 quire a powerful microscope to distinguish them, are, in 

 fact, no more than l-1200th to 1 -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. 



Cell-multiplication* The young and active cell always 

 contains a nucleus, (fig. 34, 5.) 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 parts, then the 

 protoplasm, a, 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 

 Fi S 34 - the skin of a sausage where a string is 



tightly tied around it, thus separating the two nuclei and 

 inclosing each within its new cell, which is completed 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 

 iii 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 tlic latter. 



