such species the spore-bearing generation gives rise — through the spores — 

 to a sexual generation. The sexual generation reproduces — by means of the 

 gametes — and gives rise to a spore-bearing generation. There is thus a regular 

 alternation between spore-bearing and gamete-bearing generations. These 

 two generations are called sporophyte and gametophyte respectively — that is, 

 spore plant and gamete plant. 



Among the common ferns there is a similar alternation of generations. The 

 familiar stage, with green fronds and usually a distinct underground stem, 

 is the sporophyte (see illustration, p. 387). The spores, produced in tiny 

 capsules on the under surface, are widely scattered by the wind. When a 

 spore germinates, it gives rise to a flat somewhat heart-shaped plate of green 

 cells, known as a prothallium — that is, a /^rothallus or a thallus that precedes 

 (in this case, the fern plant). Unlike the moss, the gametophyte stage is not 

 dimorphic; the prothallium bears both archegonia ^w^antheridia. The sperm 

 cells swim in water. The zygote develops into the familiar fern sporophyte. 



In What Ways Are Males and Females Different? 



The Two Kinds of Gametes In the simplest of plants and animals, 

 such as Spirogyra and Paramecium, we cannot distinguish the vegetative, or 

 growing, stage of a cell from the reproductive stage — except at, or just before, 

 the time of conjugation. Nor can we distinguish the passive, or receiving, 

 gamete from the active, or supplying, gamete except in their behavior at this 

 time: one moves and the other remains in place. As we pass on to more com- 

 plex forms, the difference becomes more pronounced. 



The swimming sperm cells of the bladderwrack and of other large algae, 

 as well as of most animals, are decidedly active. They often have very dis- 

 tinct swimming cilia, or flagella, as well as shapes that suggest movement 

 through water. In fact, when cells of this type were discovered by micro- 

 scopists, from the time of Leeuwenhoek down past the middle of the nine- 

 teenth century, they were described as "new species" of "animalcules". 



We can easily observe size, shape, food content, movement. Underlymg 

 these differences between the male and female gametes, we must assume others 

 that are related to differences in their behavior. As sperm cells swarm about 

 restlessly, they seem to turn definitely toward any tgg of the same species 

 that may be in the vicinity. We know that the eggs of certain plants influence 

 the sperms through some chemical peculiar to the species. But when one of 

 the active male gametes penetrates the egg membrane, something happens 

 instantly; for all the others immediately swim away as if they had suddenly 

 lost interest in the ^gg. But at the moment that the sperm enters, the egg 

 actually forms a membrane through which no more sperms can enter. In any 

 case, the fertilized egg does differ from the female gamete chemically. 



385 



