THE EVOLUTION OF PLANT REPRODUCTION 



55 



motile light spores capable of resisting dessication and of being 

 carried about in the air. The land environment proved a stum- 

 bling block to motile gametes also; with the result that swimming 

 sperms were gradually supplanted by male cells carried through a 

 special structure to the c^^ cell. Sexual reproduction by motile 

 sperm cells has persisted in several of the lower groups of land 

 plants — typically among the mosses and ferns. Asexual reproduc- 



EGa 



ANTHERIDIUM 



ARCHEGONIUM 



Fig. 29. — The club-shaped antheridia of a moss are packed with male 

 gametes or spermt; the flask-shaped archegonia produce the female gametes 

 or eggs. 



tion by spores, however, is so obvious in these groups that they 

 are often spoken of as "spore plants." 



A typical moss plant is a rather delicate and frail organism, 

 made up as we have seen (see p. 33) of a stem, photosynthetic 

 leaf-like organs, and rhizoids. The reproductive phase of the moss 

 life history begins with the formation of sex organs; these are 

 hidden among the leaves at the tip of the stem. In some species 

 the male and female structures are found on the same plant, in 

 others they occur upon different plants. This tendency to 

 separate the sexes becomes more frequent in the higher plantSL 



