252 



HOW CROPS GROW. 



the orange consists of cells which are one-quarter of an 

 inch or more in diameter. The fiber of cotton is a single 

 cell, commonly from one to two inches long. In most 

 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 j^^ to 5 <y of an inch in diame- 

 ter. The spores of Fungi are still smaller. The germs 

 of many bacteria are so minute as to be undiscoverable 

 by the highest powers of the microscope. 



Growth. The growth of a plant is nothing more 

 than the aggregate result of the enlargement and multi- 

 plication of the cells which compose it. In most cases 

 the cells attain their full size in a short time. The con- 

 tinuous growth of plants depends, then, chiefly on the 

 constant and rapid formation of new cells. 



Cell-multiplication. The young and active cell 



Fig 34. Fig. 35. 



always contains a nucleus (Fig. 34, 5). Such a cell may 

 produce a new cell by division. In this process the nu. 

 cleus, from which all cell-growth appears to originate, is 

 observed to resolve itself into two parts, then the proto- 

 plasm, a, begins to contract or infold across the cell in a 

 line corresponding with the division of the nucleus, until 

 the opposite infolded edges meet, like the skin of a sau- 

 sage where a string is tightly tied around it, thus sepa- 

 rating the two nuclei and inclosing each within its new 

 cell, which is completed by a further external growth of 

 cellulose, 



