122 THE EVOLUTION OF PLANTS 



plantlet, the prothallus, on which the sexual 

 organs, very definite and rather complex bodies, 

 are borne. The male organ (antheridium) is a 

 round, many-celled sac, producing a number of 

 sperms in its central tissue; the female organ 

 (archegonium) is likewise many -celled, and is a 

 flask-shaped structure, with the egg-cell at the 

 bottom, approached by a passage through the 

 neck of the flask (cf. fig. 21, B and C, p. 185). 

 Fertilisation requires the presence of water, for 

 the actively moving male cells, the spermato- 

 zoids, to swim in. If this condition is fulfilled, 

 fertilisation is fairly well insured, for the arche- 

 gonia secrete a substance (malic acid) which is 

 atttactive to the spermatozoids and guides them 

 to their goal. The egg, when fertilised, develops 

 into an embryo, which in time grows up into the 

 spore-bearing Fern plant. 



In the more advanced Spore-plants, Selagin- 

 ella (so common in greenhouses), Isoetes (a 

 related genus, mostly aquatic), and the Water- 

 ferns (of which the PUlwort, Pilularia, is our 

 native example) the sexes are distinguished at 

 an earlier stage, for the spores themselves are of 

 two kinds; there are numerous male spores 

 (microspores) of very small size, and compara- 

 tively few female spores (megaspores) of rela- 

 tively very large size. In the group of the Water- 

 ferns, only one large spore is produced in each 



