THE GREEN ALG^E (CHLOROPHYCE^E) 193 



the size of its chloroplast, it can expose more chlorophyll to 

 the light and hence do more photosynthetic work than any 

 plant yet studied in the present chapter. 



When Spirogyra cells divide, the division wall is at right 

 angles to the length of the plant. Such division results in an 

 increase in the number of cells and usually in growth in length 

 of the whole plant. Growth occurs so rapidly that within a few 

 days after the plants are first seen in the spring they become so 

 abundant that they may pollute the water in which they grow. 



FIG. 157. Spirogyra in process of spore formation 



A, conjugating cells; a and b, conjugating tubes; c, a tube from a cell which has 

 begun to conjugate with one that is already paired ; d, a second tube from a 

 paired cell. Such secondary tubes rarely develop. B, a and 6, tubes through 

 which the protoplasm is passing to unite with that of the pairing cells ; c, a tube 

 similar to c in A. Greatly enlarged 



179. Spirogyra: reproduction. Plants may become broken 

 into two or more pieces, and each piece may grow into a sepa- 

 rate plant, thus securing vegetative reproduction. At times, 

 however, there occurs a kind of reproduction quite different 

 from any that we have as yet studied in the algae. The cells 

 of two plants that lie near one another unite in pairs, this 

 union being made by means of tubular outgrowths from the 

 pairing cells (Fig. 157). These tubes meet between the old 

 cell walls, their ends fuse and form a continuous passageway 



