PLANT CLASSIFICATION 163 



number of cells, some of which form tiny outgrowths from the under 

 surface like root-hairs (called rhizoids), which anchor the prothallus 

 to the soil and aid in securing food materials. 



On the under surface likewise of each prothallus, in the region 

 of the rhizoids, are minute organs, circular in appearance, known as 

 antheridia (spermaries), in which are produced a large number of 

 sperm-cells (Fig. 83, D, E). At a little distance from the antheridia, 

 near the notch in the prothallus, are found other somewhat elon- 

 gated bodies called archegonia (ovaries). In each of these there is 

 developed a special cell known as the egg-cell (Fig. 83, F). 



178. Fertilization of the egg-cells. When the sperm-cells are 

 ripe, the antheridia or spermaries are ruptured, and the sperm- 

 cells make their way by a curious twisting motion toward the open- 

 ings on the archegonia. A single sperm-cell moves down the tube 

 of each archegonium, and penetrates the egg-cell, and the two nuclei 

 unite in the process of fertilization. (See 91.) From the fertilized 

 egg-cell develops a fern plant composed of many cells of various 

 kinds, which are all derived from the fertilized egg-cell. 



179. Alternation of generations. Thus we see that in the life- 

 history of the fern plant we have two distinct generations. The 

 first is the ordinary fern plant, which is familiar to all, and which is 

 known as the asexual generation or spore generation, because the spores 

 formed on the fronds produce the next generation (prothallus) 

 without fertilization or the union of two kinds of cells. The second 

 generation, the prothallus, is the sexual generation, because, as we 

 have seen, it can only produce a fern plant from the fertilized egg- 

 cell. In plants like the fern, in which an individual (fern) produces 

 another plant (prothallus) unlike itself, and this in turn gives rise 

 to a plant like the original (fern), we have so-called alternation of 

 generations. 



180. Suggestions for the study of the fern. No. 65. If 

 this topic is suggested for study, pupils should be encouraged to 

 collect their own material, noting the surroundings or habitat of each 

 kind of fern. They should describe the location, form, and color 

 of each of the nutritive organs, and of the fruit dots, and draw the 



