39S ELEMENTARY BIOLOGY. [CHAP. 



\vhole course. At intervals along each chromatophore 

 round bodies will be observed which appear green like 

 the rest of the band, but on careful examination are 

 each found to contain a small colourless mass of proteid 

 substance, termed the pyrcnoid. This pyrenoid is of a 

 crystalline form, usually appearing hexagonal in optical 

 section, and can be brought out more clearly by the use of 

 staining reagents. Bodies of this nature are of very general 

 occurrence in the chromatophores of the Algse. It is only 

 around the pyrenoids that starch is formed, and if the plant 

 has been exposed to the light before examination, the 

 pyrenoid will be found to be surrounded by a layer of small 

 starch-grains often present in so great a mass that the band 

 is distinctly swollen at the places where they occur. 



As already mentioned the greater part of the interior of 

 the cell is occupied by a single large vacuole, containing 

 cell-sap, i.e. water in which certain inorganic salts, and 

 certain organic substances, such as sugar and acids, are 

 dissolved. 



In addition to the structures already described each cell of 

 Spirogyra contains a conspicuous nucleus. Its position varies 

 somewhat in different species, for in some cases it is placed 

 in the middle of the cell, while in others it is nearer to 

 the external wall. Forms with a central nucleus are more 

 convenient for observation. The shape of the nucleus also 

 shows certain specific differences. A very common form is 

 that of a biconvex lens, the edge of the lens being turned 

 towards the lateral walls of the cell, and the circular faces 

 towards its ends. The nucleus is surrounded by a layer of 

 protoplasm, from which long processes radiate which 

 traverse the vacuole and are attached at their further ends 

 to those parts of the chromatophores in which the pyrenoids 

 are situated. 



