The Origin of Land Plants 107 



Heterospory. In the horsetails, and in certain 

 ferns, the gametophyte is unisexual, the male plants 

 being usually smaller than the female, which of 

 course has to support the young sporophyte. This 

 separation of the sexes is the first step in the direc- 

 tion of what is known as " heterospory," i.e., the 

 production of two sorts of spores, large and small, 

 developing respectively female and male gameto- 

 phytes. The large spores are known as macrospores 

 or megaspores, the small ones, microspores. In all 

 cases of heterospory, the gametophytes are much 

 reduced in size, this being especially true of the 

 males, which may consist of a few cells only, and 

 have their whole development completed within less 

 than twenty-four hours. The most marked case of 

 this is shown in the water fern, Marsilia. In 

 this plant only one spore comes to maturity in 

 the megasporangium, while in the microsporangium 

 all of the spores mature. If the ripe spores are 

 placed in water of a suitable temperature, growth 

 begins at once, and in a common Calif ornian spe- 

 cies, within from fifteen to twenty hours the gameto- 

 phytes are completely developed and fertilization has 

 been effected. The spermatozoid fertilizes the 

 archegonium, and from the egg the young sporo- 

 phyte, which closely resembles that of the typical 

 ferns, is quickly developed. 



Heterospory has arisen in several quite unrelated 

 groups of Pteridophytes, and it is clear from a study 

 of the fossil forms that a number of the hetero- 



