86 PLANT-ANIMALS [CH. 



characteristic of the cells of many of the lower algse 

 (Fig. 21, p. 123). 



If a green cell is treated with a solution of iodine, 

 the nucleus and the pyrenoid are stained brown, and 

 round the latter a thin, blue, granular layer may be 

 distinguished. This layer is known as the starch 

 sheath. As the action of the iodine continues, 

 minute, lens-shaped starch grains distinguished by 

 their blue colour may be seen lying in the chloro- 

 plast. Unlike algal cells in general, the green cells 

 in the body of C. roscoffensis have no cellulose wall, 

 but are bounded each by an elastic layer of protoplasm. 



The yellow-brown cells of C. paradoxa are built 

 on somewhat different lines. Each cell contains a 

 number of irregularly oval, or polygonal, yellow- 

 brown, discoidal chloroplasts which occupy about 

 half of the cell (Fig. 1 8). The other half of the yellow- 

 brown cell consists of clear, transparent, vacuolated 

 protoplasm. By suitable treatment, involving the 

 dissolution of the pigment, a nucleus may be made 

 out, slung in the centre of the cell by threads of 

 protoplasm which stretch from the periphery. When 

 the cells are treated with alcohol, the yellow-brown 

 pigment is dissolved away and green chlorophyll, 

 previously screened by the yellow-brown pigment, is 

 seen in the chloroplasts. The reaction is useful in 

 that it enables us to distinguish the chloroplasts 

 of the yellow-brown cells from the orange-coloured 



