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THE POPULAR SCIENCE MONTHLY, 



sperm. The male proth alius is reduced and rudimentary, and one cell of 

 the pollen-grain, representing the antheridium, produces a tube instead 

 of antherozoids. The endosperm is the female prothallus, and in it the 

 germ-cell develops. The asexual generation is the plant that grows 

 from the fertilized germ. In the angiosperms the sexual generation is 

 reduced to its simplest form, namely, a single cell for the male part, 

 and one or a few cells for the female. 



We thus see that the alternation of generation, viewed in the light 

 of its presentation among mosses and ferns, practically disappears in the 

 higher flowering plants. The sexual generation is so reduced and 

 merged with the asexual that the two seem to become one, and, were it 

 not that the gradual simplifying of this generation may be traced, it 

 would not be thought to exist. 



If we recapitulate, in the reverse order, it is easy to evolve a con- 

 spicuous, independent plant from the single-celled pollen-grain, and a 

 similar self-supporting plant from the simple embryo sac. The first 

 step back is to the gymnosperms, where the ovules are not in ovaries, 

 and the embryo sac has a rudimentary prothallus in the endosperm. 

 The pollen-grain is made up of more then one cell. From this group we 

 pass to the Selaginellw, in which we have the female spore, with its rudi- 

 mentary prothallus, and the smaller male spore, having one cell for the 

 prothallus, the remaining ones forming the antherozoids. Descending 

 to the Rhizocarpem, both the spores increase in size and complexity. 

 Further back we come to the higher orders of ferns, with but one kind 

 of spore, the small prothallus, bearing both sexual organs. The lower 

 orders have this sex-bearing generation much more developed. In the 

 mosses and Hepaticm the sexual generation surpasses the asexual in 

 size and complexity. 



The relative size of the two generations might be represented to the 

 eye by drawing a rectangle Avith a diagonal, Fig. 18. One triangle 



U! 



< 

 u 



< 



Q. 



lil 



SEXUAL GENERATION! 



ASEXUAL GENERATION 



U) 



Fig. 18. 



would indicate the size and complexity of the sexual generation, while 

 the other represented the asexual generation. The sexual generation 

 diminishes from the Hepaticm to the angio-sperms, while the asexual 

 generation increases. 



The engravings here employed are from treatises on botany by 

 Sach, Prantl, and Bessey, to whom the writer is also indebted for many 

 of the facts brought together. 



