36 PLANT-LIFE 



They are embedded in the primordial utricle, and occur 

 in the form of spiral bands with irregular edges. The 

 chlorophyll' pigment renders them a beautiful, delicate 

 green. The chloroplasts in a single cell vary in number, 

 according to species, and even in the cells of a single 

 species or a particular thread the number is not 

 always constant; it ranges from one to ten. Pyrenoids 

 (see p. 32), arranged in a single row, occur at intervals 

 in the chloroplasts. From this description it will be 

 observed that the Spirogyra cell is a well- organized 

 structure. 



Spirogyra is counted a multicellular plant, but we 

 might reasonably regard each filament as a string of 

 attached one-cell plants, for each cell is similar, and 

 able to provide for its own needs. A filament may break 

 and resolve itself into separate cells, each one being able 

 to exist independently, and, in time, give rise to a new 

 filament. Growth is achieved by cell-division. In 

 this process the nucleus divides into two similar daughter- 

 nuclei; a septum of cellulose — the organic substance of 

 the cell-walls of the majority of plants — is gradually 

 formed. This divides the cell into two daughter-cells, 

 each one containing one of the daughter-nuclei. This, 

 of course, spells vegetative growth, not reproduction. 



In Spirogyra we have a plant which, normally, is 

 entirely sexual in reproduction. The sexual process is 

 most interesting. It happens in two ways: in the first 

 the contents of two adjacent cells of a filament recede 

 from the cell-walls and form rounded masses in the 

 centres of their respective cells ; then an opening is made 

 in the septum separating the cells, and one of the 

 rounded masses of cell-contents passes to the other, 



