ALTERNATION OF GENERATIONS. 



205 



times simply in the epithelium lining the body walls. There may be a 

 long series of generations producing and produced in this way, and these 

 are often unlike one another. Fluke, embryo, sporocyst, redia, and cer- 

 caria, are all markedly different in structure, though embryo changes into 

 sporocyst, and cercaria into fluke. 



This alternation between sexual reproduction with the usual fertilisation, 

 and reproduction by means of special cells which yet require no fertilisa- 

 tion, prevails in many plants, e.g.^ ferns and mosses. From a fertilised 

 egg-cell the ordinary fern-plant, with which everyone is familiar, develops. 

 But this is quite asexual, if we mean by that that it is neither male nor 

 female, and that it produces neither male nor female elements. At the 

 same time it produces special reproductive cells, — not egg-cells exactly, any 

 more than those within the sporocyst were, but yet able to develop of 

 themselves into a new organism. This is not another fern-plant, however, 

 but an inconspicuous green organism, much less vegetative, and sexual. 

 The so-called " spore" formed on the leaves of the sexless fern-plant falls 

 to the ground, develops a " prothallus," which bears male or female organs, 

 or both. An egg-cell is fertilised by a male element, and the conspicuous 

 vegetative fern-plant once more arises. The formula is therefore as 

 follows : — 



Where A = sexless vegetative fern-plant ; 



sp. = the parthenogenetic special reproductive cell or spore ; 

 S = the sexual inconspicuous " prothallus," with male and female organs. 



Now take the history of a moss. Unlike the fern, the more con- 

 spicuous "moss-plant" is sexual. It bears male and female organs, and 

 an egg-cell is fertilised by a male element. The fertilised egg-cell, how- 

 ever, does not lose its hold of the mother plant, but grows like an encum- 

 bering parasite upon it. Obviously, then, it does not give rise to another 

 " moss-plant." The result of the fertilised egg-cell is a tiny sexless stalk, 

 which bears on its apex the special reproductive cells or spores with which 

 we are now familiar. In other words, the fertilised egg-cell develops into 

 a parasitic spore-bearing generation. The "spores" fall into the ground, 

 as they did in the fern, and there grow into a usually thread-like structure, 

 from which the sexual moss-plants are budded off. If we do not emphasise 

 the transitional thread-like stage, — the protonema as it is called, — the 

 formula is as follows (see also fig. p. 201) -. — 



Where A= inconspicuous sexless parasitic generation upon the " moss-plant. 



sp. = the special parthenogenetic reproductive ceil or spore produced by A. 

 S=the conspicuous sexual "moss-plant," budded from the threads 

 developed from the spore. 



