CONJUGATION OF GAMETES 33 



the absorption of carbon dioxide and the evolution of oxygen gas 

 which accompany photosynthesis. In darkness, however, the 

 gaseous interchange which forms the essential feature of respira- 

 tion can readily be detected, carbon dioxide, produced by oxidation 

 of the protoplasm, being given off and oxygen taken in. 



No special organ of excretion has been observed in Haemato- 

 coccus, though a contractile vacuole occurs in closely allied forms, 

 and waste products must simply diffuse into the surrounding 

 water through the permeable cell- wall. 



We have already seen that Haematococcus multiplies itself by 

 simple fission within the old cell-wall. This process usually 

 results immediately in the production of four new individuals. 

 Under favourable circumstances it may be repeated very rapidly, 

 without the organism going through any true resting stage, so 

 that in a short space of time the number of active zoospores 

 may be very largely increased. The individuals thus produced 

 are usually all of the same form, and ultimately of the same size, 

 as the parent. Occasionally, however, a somewhat different 

 process of multiplication takes place. Instead of dividing into 

 four relatively large zoospores a resting individual may divide into 

 thirty-two or sixty-four much smaller "microzooids" (Fig. 5, D), 

 which differ from the ordinary active form in the absence of the 

 characteristic cell- wall with its underlying vacuole. 



The microzooids (Fig. 5, E) swim actively about by means ol 

 their flagella. Sooner or later, however, they come together in 

 pairs (Fig. 5, F, G), and the members of each pair fuse completely 

 with one another to form a single individual (Fig. 5, H) with 

 four flagella, which presently loses its flagella, secretes around 

 itself a thick cell-wall, and enters upon the resting state 

 (Fig. 5, J, K). From this resting individual new generations will 

 be produced by the ordinary method of division into zoospores. 



We have here an excellent illustration of what is usually 

 termed sexual reproduction, the essential feature of which is the 

 union or conjugation of two sexual cells or gametes (in this case 

 the microzooids) to form a single cell, the zygote, which is the 

 starting point of a fresh series of cell generations. This important 

 process will be discussed more fully in a subsequent chapter. 



We have spoken of Amoeba as an animal, and, as we have seen, 

 many people regard Haematococcus as a plant. We must next 

 endeavour to find out what it is that really differentiates a plant 

 from an animal. Of course amongst Jdie more highly organized 



B. . y> 



