xix INTERSTITIAL GROWTH 197 



confined to the primordial utricle ; towards the centre of the 

 vacuole is a small irregular mass of protoplasm connected to 

 the peripheral layer by extremely delicate protoplasmic 

 strands. Imbedded in this central mass is the nucleus (nu\ 

 which has the form of a biconvex lens and contains a distinct 

 nucleolus. 



The chromatophores differ from anything we have yet 

 considered, having the form of green spiral bands (chr), of 

 which each cell may contain one (D 1 ) or two coiled in oppo- 

 site directions (A). Imbedded in the chromatophores are 

 numerous pyrenoids (pyr, see p. 27), to which the strands 

 of protoplasm proceeding from the central nucleus-containing 

 mass can be traced. 



The process of growth in Spirogyra is brought about by 

 the binary fission of its constituent cells. It takes place 

 under ordinary circumstances during the night (i i 12 P.M.), 

 but by keeping the plant cold all night may be delayed until 

 morning. 



The nucleus divides by the complicated process (karyo- 

 kinesis) already described in general terms (p. 67), so that 

 two nuclei are found at equal distances from the centre of 

 the cell. The cell-body with its chromatophores then begins 

 to divide across the centre (B 1 ), the process commencing 

 near the cell-wall and gradually proceeding inwards : as it 

 goes on cellulose is secreted between the halves of the 

 dividing protoplasm so that a ring of cellulose is formed 

 lying transversely across the middle of the cell, and in con- 

 tinuity externally with the wall (B 2 ). The ring is at first very 

 narrow, but as the annular furrow across the dividing cell- 

 body deepens, so the ring increases in width, until by the 

 time the protoplasm has divided it has become a complete 

 partition separating the newly-formed daughter-cells from 

 one another. 



