260 



XX. STRUCTURE OF THE THALLOPHYTA. 



shows, a number of chromatophores are present, which, in 

 lateral contact, occupy the exterior of the cell-contents. Where 



their contact is not com- 

 plete, the colourless 

 embedding protoplasm 

 comes into view. More 

 or less in the middle of 

 the cell, but not usually 

 visible without the help 

 of reagents, lies the 

 nucleus, with its nuc- 

 leolus. The cells have 

 a thin wall, which stains 



violet with chlorzinc 

 FIG. 98. Protococcus viridis, after treatment with . -,. AT n 



potassium-iodide-iodiue. In /> the cells on the left iodine. JNumeroUS cells 

 have just divided (x 540). are usua ll y i n course 



of bipartition by means of a partition wall, which cuts the spheri- 

 cal cell into halves (Fig. 98, D). The divisions of adjoining 

 cells take place in planes either parallel or cutting one another 

 at right angles. The daughter-cells, becoming rounded off, soon 

 pass out of union with one another (C, F) ; they may remain for 

 some time clinging together, or else become at once separated. 

 If the cells are treated with potassium-iodide-iodine, the nuclei 

 show up clearly (our figures were sketched from such prepara- 

 tions). In each nucleus the nucleolus is then clearly visible. In 

 the cells which have just arisen by division, the nuclei lie against 

 the young partition wall (D). The iodine solution shows small 

 starch grains in the chromatophores, but no pyrenoids. 



We have said that this alga is by some considered to be only 

 a stage in the life history of some higher form, probably one of 

 the Volvocinese. If some of this material is placed in water and 

 exposed to the light, some of the cells will enter upon another 

 phase of their existence, becoming motile by means of cilia. 

 More suitable for microscopical study, however, is another form 

 of Protococcus, commonly known as Protococcus (or Hcematococ- 

 cus) pluvialis. This Protococcus is found very widely distributed 

 in rain-water, in the mud at the bottom of open water-butts, in 

 roof -gutters, etc. 



Structure of Nostoc. We will now turn our attention to one 

 of the Nostocaceae, which has somewhat curious relations with 

 another plant, which is widely cultivated in botanical gardens, 



