240 APPLIED BIOLOGY 



seed-plants have two generations alternating in their life- 

 history. One of these generations (the sporophyte or spore- 

 forming stage) is the ordinary seed-plant (e.g., an oak tree) ; 

 and its spores are pollen-grains and ovules. The oak tree 

 then corresponds to the fern-plant in that each is the spore- 

 producing stage. The other generation (the gametophyte, 

 which produces the sex-cells) in seed-plants is the pollen- 

 tube, in which is formed the sperm-cell, and a microscopic 

 structure (the embryo-sac) in the ovule, in which is formed 

 the egg-cell. The development of the sperm-cell in the pollen- 

 tube and of the egg-cell in the embryo-sac of seed-plants is 

 believed to be a great modification of the formation of sperm- 

 cells and egg-cells in the sex-organs of prothallia of the lower 

 plants. In ferns the spores are all alike, but in seed-plants 

 there are two kinds, the smaller microspores or pollen- 

 grains, and the larger megaspores or ovules. A minor differ- 

 ence is that the spores of ferns grow extensively, forming 

 prothallia, which later produce sex-cells, but seed-plants 

 develop the sex-cells by cell-divisions inside the spores. Thus 

 in both ferns and seed-plants the sex-cells come more or less 

 indirectly from spores, and the united sex-cells of the two kinds 

 develop into ferns and seed-plants, which in their turn form 

 spores again. 



228. Allies of Ferns. In addition to the plants which 

 are easily recognized as ferns, and of which there are numerous 

 species in the United States, there are a number of closely 

 related fern-like plants. These are the water-ferns, the horse- 

 tails (Equisetum), and the club-mosses (lycopods). Speci- 

 mens (fresh or in formalin) should be exhibited in order to 

 give general acquaintance. If time permits, special text- 

 books of botany should be consulted as guides to very brief 

 study of specimens of these plants. 



All the fern-like plants taken together constitute a group 

 known as the Pteridophytes (Pteridophyta), one of the pri- 

 mary divisions of the plant kingdom (see table in Chapter 



