238 APPLIED BIOLOGY 



accomplish fertilization of the egg-cell in an ovule of a flower. 

 The only noteworthy difference is that the sperm-cell of the 

 fern, like the sperm-cell of animals, is adapted for swimming 

 in fluids, while the fertilizing cell derived from a pollen-grain 

 can reach the egg-cell in a pistil only through a pollen-tube. 

 It appears that the development of pollen-tubes in the seed- 

 plants is an adaptation to the dry conditions surrounding most 

 flowers. Even water-lilies and other aquatic seed-plants 

 have stems which hold the flowers above the water so that 

 there could be no opportunity for a swimming cell to move 

 from an anther down into the ovary. The dry conditions in 

 all flowers of seed-plants are well adapted to the transfer of 

 the fertilizing cell or sperm-cell from the anther to the egg- 

 cell by means of (1) pollination, and (2) growth of a pollen- 

 tube down the style to the ovule. 



The male and female sex-cells are often called gametes, 

 (meaning bodies which unite with similar bodies) ; hence in 

 some books the egg-cell or ovum is named female gamete, 

 and the sperm-cell is the male gamete. 



Development of the Fertilized Egg-cell. The fertilized 

 egg-cell (also called gametospore, or oosperm, meaning 

 combined egg and sperm) of the fern, begins development 

 soon after fertilization. By repeated cell-division similar to 

 that already described ( 59), the egg-cell is divided into a 

 large number of cells, which forms an embryo, and this grows 

 into a plantlet as shown in Fig. 72, G. In it can be seen 

 cotyledons, stem, and root. Soon afterward, the prothallium 

 withers, and the young fern plant becomes independent. It 

 grows to maturity, produces spores on its leaves, and these 

 spores in turn start another cycle or life-history. 



226. Alternation of Generations in Ferns. It is evident 

 from the above account that the complete life-history of a 

 fern is composed of two individuals quite different in appear- 

 ance and other characteristics, especially in the method of 

 reproducing. The fern plant reproduces by means of spores 



