ORIGIN OF SEX IN PLANTS 89 



attraction is mutual. The process of conjugation is followed as 

 usual by cell-division on the part of the zygote, which in this 

 case results in the formation of a small number of comparatively 

 large spores, enclosed in tough protective envelopes. From 

 these spores new individuals are produced which, under favour- 

 able circumstances, commence the life cycle afresh. It is 

 extremely interesting to observe that we have here, in a 

 unicellular Protozoon, as complete a sexual differentiation of 

 the gametes as we meet with in any of even the most highly 

 organized plants and animals. 



We must now briefly notice the sexual phenomena exhibited 

 by those interesting Protista which we have already had occasion 

 to refer to in Chapter IV. under the name Phytoflagellata. It 

 will be remembered that in Haematococcus (Fig. 5) the process of 

 simple fission sometimes results in the production of a relatively 

 large number (32 64) of small individuals instead of the usual four 

 comparatively large ones. These small individuals are specialized 

 gametes, differing from the large ones not only as regards size 

 but also in the absence of the characteristic cell-wall of the latter. 

 There is, however, no differentiation into male and female. 

 Conjugation is of the isogamous type and produces a zygote 

 which grows into an ordinary resting cell which will presently 

 begin to multiply actively by ordinary fission. 



We have also seen that this organism forms the starting point 

 of a series of forms, represented by the genera Haematococcus, 

 Pandorina, Eudorina and Volvox, which illustrate progressive 

 stages in the process of colony formation. The same series also 

 shows us very clearly the differentiation between male and female 

 gametes the origin of sex in the vegetable kingdom. It will be 

 remembered that Pandorina forms colonies of sixteen or thirty- 

 two cells enclosed in a common envelope (Fig. 10, A). A sexual 

 multiplication is effected by each cell of the colony dividing into 

 2, 4, 8, and finally 16 or 32, which form a daughter colony within 

 the parent, to be liberated presently by softening of the parental 

 envelope. Occasionally, however, the individual cells of a colony 

 divide each into eight gametes. These are small cells, each with 

 a pair of flagella, which escape and swim about separately. They 

 exhibit no clear distinction into male and female, but some are 

 comparatively large, some small, and some intermediate in size. 

 They conjugate in pairs (Fig. 10, B), and the two members of a 

 conjugating pair are often, though apparently not always, of 



