THALLOPHYTA. 629 



plasm covered by a delicate cell-wall of cellulose. At first sight the whole body 

 appears green, but on careful examination it will be seen that the green colouring- 

 matter is really confined to a layer on the surface of the body. This chlorophyll- 

 layer or chromatophore is specially thickened at one end (the posterior end) of the 

 body, and a small, clear, spherical body (the pyrenoid) is often to be seen inclosed 

 in this thickened portion. Round the sides of the body the chromatophore is 

 thinner, and towards the anterior end it stops, leaving a small area of colourless 

 protoplasm to occupy the front extremity of the organism. This is in connection 

 with another small mass of colourless protoplasm which occupies the hollow of the 

 cup-shaped chromatophore. 



A minute red speck is visible at one side of the body, situated sometimes on the 

 surface, sometimes at the limit between the chromatophore and the central colour- 

 less protoplasm. The rotation of the Ghlamydomonas about its antero-posterior 

 axis, which accompanies the jerky, forward movement (c/. vol. i. p. 29) can be well 

 traced by the aid of this eye-spot, which can be seen to be carried round and 

 round as the organism turns on its axis. The means by which the CJdamydomonas 

 moves are not at first sight apparent. But when the cell has come to rest, and 

 especially if it has been killed and its protoplasm fixed with a dilute solution of 

 iodine, a pair of very delicate protoplasmic filaments, called flagella, can often be 

 seen projecting from the colourless anterior spot of protoplasm. It is by the con- 

 tinual lashing of these flagella that the Ghlamydomonas is pulled through the 

 water. The iodine will also bring into view a small spherical nucleus, stained dark 

 brown, situated in the colourless central protoplasm, and will stain the neighbour- 

 hood of the pyrenoid a dark blue. This last phenomenon is due to the formation 

 of starch by the protoplasm round the pyrenoid. The exact part played b}^ the 

 pyrenoid is very obscure, but there can be no doubt that it influences in some way 

 the formation or deposition of starch by the protoplasm. 



The movements of Chlam^ydomonas are often in direct relation to light, the 

 organisms moving towards a source of light of moderate intensity. Thus if a glass 

 vessel filled with water containing Chlamydomonas be placed in a sunny window, 

 there will be a general movement of the swarming cells to the sunny side of the 

 vessel, causing the water to become much greener on that side. There is some evi- 

 dence that it is the eye-spot which enables the organism to perceive the direction 

 from which light is proceeding. Nearly all the motile Protococcoideae, as well as 

 zoospores and gametes which are sensitive to light, possess an eye-spot, and it has 

 been thought that the eye-spot-pigment (a substance called hcematochrome) may act 

 in the same way as the visual purple in the retina of the eye. 



If specimens of Chlamydomonas be kept for a day or two in water, some of them 

 will often be found to have divided to form daughter individuals, which still remain 

 inclosed within the cellulose membrane of the mother. This division is preceded by 

 the drawing in of the flagella. The protoplasm of the body then withdraws itself 

 from the wall, and divides transversely to form two roundish masses. Each of the 

 latter may either at once put on a cell-wall and develop flagella, or it may divide 



