ALTERNATION OF GENERATIONS IN PLANTS 101 



spermatozoa, owing to the great amount of cytoplasm which they 

 contain. They are liberated by rupture of the oogonium and 

 discharged through the opening of the conceptacle on to the 

 surface of the plant. There they are found by the spermatozoa, 

 which swarm around them in large numbers, endeavouring to 

 conjugate (Fig. 47, c). Finally a single spermatozoon succeeds 

 in boring its way into each large egg cell, and fertilization is 

 effected by the union of the male and female nuclei. The zygote, 

 well supplied with food material by the egg cell, begins to undergo 

 cell-division immediately, forming a multicellular embryo 

 (Fig. 47, d) which attaches itself by roots and grows into a plant 

 resembling the parents. 



Here we have a perfectly typical case of differentiation of the 

 gametes or germ cells into large passive female ova and small 

 active male spermatozoa, and conjugation is anisogamous, the 

 ovum being " fertilized " by the spermatozoon. 



In the ferns, mosses and other more highly organized plants 

 a new complication is introduced by the fact that two distinct 

 forms of the plant alternate with one another in the life cycle. 

 In one only of these forms, known accordingly as the gametophyte, 

 does a sexual process occur ; the other, known as the sporophyte, 

 reproduces by means of unicellular spores, which are produced 

 asexually and develop into new individuals without any process 

 of conjugation. The gametes or germ cells, borne on the gameto- 

 phyte, on the other hand, conjugate, and the zygote develops, 

 not into another gametophyte but into a sporophyte, while, 

 conversely, the spores produced by the sporophyte develop into 

 gametophytes. 



This alternation of sexual and asexual generations is a 

 phenomenon of very wide-spread occurrence in the vegetable 

 kingdom, and, as we shall see in our next chapter, something of 

 the same kind occurs also in certain multicellular animals. 



Take, for example, any ordinary fern. The conspicuous plant 

 (Fig. 48) is the sporophyte. It is very highly organized and shows 

 the typical differentiation into root, stem and leaf met with in all 

 the higher groups of the vegetable kingdom. Some or all of the 

 leaves sooner or later produce on their lower surfaces sporangia 

 (Fig. 48, A, C), little sac-shaped structures in which the spores 

 arise by division of mother cells into fours. These spores are 

 liberated, by rupture of the sporangia, in the form of fine brown 

 dust, which may be carried to considerable distances by the wind. 



