54 PLANTS AND MAN 



ferentiated, exhibiting differences in appearance and behavior. 

 This can be seen in another pond scum, Oedogonium (fig. 28). 

 Certain cells of the filament transform their protoplasmic con- 

 tents into single large gametes, each of which completely fills 

 the cell and is incapable of leaving it. Such a large immotile 

 gamete is not only a potential reproductive cell but also a store- 

 house of reserve food. Other cells of the Oedogonium filament 

 produce smaller gametes which possess flagella and look more 

 like the gametes of Ulothrix. When released from the cell, these 

 motile gametes swim about and eventually one of them chances 

 to find the opening provided in the cell which contains the large 

 gamete. Entering through this, the small gamete fuses with the 

 larger one to form a single cell. The large immotile gamete is 

 known as an egg, and the small motile gamete is a sperm. In all 

 higher plants, as well as in animals, sexual reproduction is by 

 gametes of this type. The fusion of the two gametes thus seen 

 in a primitive fashion in Oedogonium is known as fertilization — 

 the goal of every sexual reproductive act. It is a step farther in 

 specialization, beyond the fusion of similar gametes. The fertil- 

 ized ^^'g, released by the breaking down of the cell wall surround- 

 ing it, undergoes cell division and grows into a new plant. 



Even greater specialization is to be seen in the brown and red 

 seaweeds, where the gametes are produced in special structures, 

 rather than in ordinary vegetative cells. These structures are 

 known as sex organs, and one — the female sex organ — produces 

 the eggs, while the other, or male sex organ, produces the sperms. 

 The ^gg remains in the female sex organ, motility being char- 

 acteristic of the male gamete only. 



The Reproductive Cycle of a Moss 



Reproduction among land plants is a modification of the 

 inherited pattern laid down in the foregoing primitive types of 

 reproduction. But the transition to land living made certain 

 changes imperative in both sexual and asexual reproduction, to 

 adapt plants to an environment where movement of reproductive 

 cells could take place only with great difficulty. Asexual repro- 

 duction by swimming spores, for example, is possible only in a 

 liquid medium; thus spore formation on land must involve im- 



