253 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 iaict, no more than -j^^^ to ^^ of an inch in diame- 

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

 of many bacteria are so minute as to be undisooTcrable 

 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 actire cell 



Fig 34. ^ig- 35. 



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

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

 cleus, from which all cell-gTowth 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. 



