100 ELEMENTS OF STHUCTURAL BOTANY. 



soft body called the nudcus. The space between the 

 nucleus and the lining of the cell is filled with a thick- 

 ish liquid called prutuplamn, and the microscope re- 

 veals to us the fact that, as long as the cells are liriiuf 

 cells, a circulation or current is constantly kept up in 

 the protoplasm of each. To this curious movement the 

 term cyclosu has been applied. As the cells become 

 older, the nature of their contents is altered by the 

 introduction of w^atery sap, in which other substances 

 are found, noi*ably starch, sugar, chlorophyll (to which 

 leaves owe their green colour), and crystals (raphides) 

 of various salts of lime. Tlie substance of which the 

 cell-wall is composed is called cellulose, and is a 

 chemical compound of carbon, hydi-ogeu, and oxygen. 

 In the protoplasm nitrogen is found in addition to the 

 three elements just mentioned. 



IGJl. Tlie (jrontli of u plant consists in the niuUiplication 

 of its cells. Every plant begins its existence with a 

 single cell, and by the repeated division of this, and the 

 growth of the successive sections, the whole fabric of 

 the plant, whether herb, shrub, or tree, is built up. The 

 division of a cell is accomplished by the formation of a 

 partition across the middle of it, the nucleus having 

 previously separated into two pieces. The jiartition is 

 formed oat of the lining of the cell. Each half of the 

 cell then enlarges, and, when its full size is attained, 

 divides again, and so on, as long as the cells are living 

 cells. 



165. But in order to increase their size, food of some 

 kind is essential. Growing plants supply this to their 

 cells mainly in the form of sap, which is taken in by 

 the root-fibres, and mt de suitable, or elaborated, or 

 assimilated, by chemical action in the plant itself. By 



