92 THE PLANT CELL. 



In a living Spiro(jijra cell it is often possible to detect small 

 granules in the cytoplasm in the vicinity of the pyrenoids, and, 

 by carefully focussing these, and cutting off the peripheral rays 

 of illumination to render them sharper in definition, it will be 

 seen that they are vibrating rapidly to and fro. It is probable 

 that this is not quite the same sort of movement as the well- 

 known Brownian vibration of small particles in the protoplasm, 

 which is a physical phenomenon, but is evidence of protoplasmic 

 activity, since the vibrating particles are situated chiefly over the 

 pyrenoids, and the bulk of these latter bodies is protoplasmic 

 in nature, each pyienoid in fact being looked upon as a 

 plastid. 



If Schulze's solution be used as a reagent the cell-wall will stain 

 blue, showing that it is composed of pure cellulose. The other 

 reactions are the same as those noticed in a, ^, and 7. 



For the complete study of Spirogi/ni very instructive prepara- 

 tions may be made by first fixing filaments in very dilute 

 Flemming's solution or chromic acid (i per cent.). The filaments 

 are then washed in distilled water, treated with alcohol for a 

 few minutes, and stained with the EhrUch-Biondi triple stain 

 (composed of methyl-green, fuchsin, and orange G). By this 

 method the cytoplasm is stained pink, the nucleus green, and the 

 chlorophyll band and pyrenoids reds of different shades. 



The cells of which a filament of Spirogyra is made up form 

 very good examples of typical assimilating cells, in which the 

 production of starch (or sugar) forms a large part of the pro- 

 cesses of assimilation. Moreover, this starch is only formed in 

 the presence of light, as may be easily demonstrated by growing 

 filaments in the dark for some days, when the cells will, if 

 treated with iodine solution, show the pyrenoids devoid of any 

 peripheral starch-rings. The nitrogenous substances requisite for 

 formation of the proteid and amine parts of the cytoplasm mole- 

 cule are derived from tlie dilute solution of nitrites and nitrates 

 in the surrounding water, these being, together with water itself 

 and other salts, assimilated mostly during the absence of light. 

 The oxygen necessary for respiration is also derived from the 

 surrounding water, in which traces of oxygen are dissolved ; and, 

 possibly, some of the oxygen evolved from the cell during the 

 assimilation of carbon dioxide is dissolved in the water and used 

 again for purposes of respiration. Carbon dioxide exists in the 



