REPRODUCTION: ALTERNATION OF GENERATIONS 271 



plant to the soil and giving it nourishment. It is called a 

 prothallium (Lat. pro = before + ihallus = branch) and one 

 would never suspect that this little plant had anything to do 

 with the fern which produced it. We rarely see the prothallia 

 of the fern, not because they are not abundant, but because 

 they are so small and grow so closely to the ground that they 

 do not attract attention. They may be found without much 

 difficulty, however, by carefully searching for them. One of 

 the easiest places to find them is on the outside of the moist 

 earthen flower pots in a greenhouse where ferns are abundant. 

 After the prothallium has reached its full growth an exami- 

 nation of its under surface with a microscope shows that it 

 in turn is getting ready to carry on a process of reproduction. 

 On the under surface may be found several little projections 

 (Fig. C), too small to be visible to the naked eye but clearly 

 made out with the microscope. They are of two kinds, one 

 lying among the rhizoids near one edge of the leaf, an, and the 

 other lying near the other edge, some' distance from the rhi- 

 zoids, ar. The latter are slightly elongated, with an opening 

 at the free end, and a little canal extending down the middle: 

 they are called archegonia (Gr. arche- = beginning + gonos = 

 birth) ; Figs. D and E. At the base of each archegonium is a 

 single egg, o. The other protuberances, lying near the edge of 

 the leaf among the rhizoids, are called antheridia (Gr. antheros 

 = flowery) ; Figs. F and G. They are more rounded in shape, 

 not so long as the archegonia, and their contents are quite 

 different. Instead of containing a single egg, the whole con- 

 tents of an antheridium divides up into a large number of 

 parts. Eventually an opening makes its appearance at the end 

 of the antheridium, and these minute bodies emerge and prove 

 to be sperms, spm (sometimes called spermatozoids). The 

 fern prothallium grows only on moist surfaces and clings 

 so closely to the ground that in times of rains or heavy dew 

 its under surface is likely to be covered with water. Each 

 sperm bears a tuft of swimming flagella, which lash to and fro 



