STRUCTURE OF CHLAMYDOMONAS 181 



cellular expansion (Figs. 108, 109). The further classification of the 

 Vegetable Kingdom (cf. Appendix, IX) is based mainly on the 

 methods of reproduction and the structure of the reproductive 

 organs, as will become apparent in the course of the subsequent 

 'chapters. 



A considerable number of the simplest types of plants 

 consist of a single cell, and such minute organisms, by their 

 abundance, often cause the green colouration of small stagnant 

 pools. Some of the forms commonly responsible for this 

 phenomenon belong to the genus Chlamydomonas , which will 

 serve to illustrate the structure of these plants, one of whose 

 most striking characteristics is a power of movement from 

 place to place, usually associated with animals. 



If a drop of water containing species of this genus be examined 

 under the microscope, the individuals will be seen to move 

 rapidly across the field of view. Each consists of a spherical 

 or oval cell, about one-fiftieth of a millimetre in diameter. The 

 cell is bounded by a thin wall which is often produced into a slight 

 colourless papilla at the front end (Fig. 96, d), i.e. that which is 

 foremost during movement. There is a single large chloroplast, 

 having the shape of a deep cup with a very thick base and with 

 the opening directed forwards (Fig. 96, d, ch.). Within the thick- 

 ened part is embedded a pyrenoid (cf. p. 7), surrounded by a 

 sheath of small starch-grains (Fig. 96. p.), but after active 

 assimilation starch may also be found stored in other parts 

 of the chloroplast. Adjacent to the rim of the latter lies a small 

 red speck or streak of modified cytoplasm known as the eye-spot 

 (stigma) (Fig. 96, s.), which is believed to be concerned with the 

 perception of light intensity (cf. below). A single nucleus (n.) 

 lies in front of the pyrenoid suspended by cytoplasmic strands 

 within the cavity of the chloroplast. 



If a stationary individual be observed under the high power 

 of the microscope, a pair of small circular highly refractive 

 vacuoles will be recognised in the clear cytoplasm at the front 

 end (Fig. 96, C.V.) ; when these are watched closely they will be 

 seen, alternately, to enlarge gradually and then suddenly to 

 collapse. Similar contractile vacuoles occur in many unicellular 

 plants, as well as among the lower animals, and they probably 

 serve for the excretion of waste products to the exterior. 



