REPRODUCTION AND DISPERSAL 885 



A somewhat remarkable situation occurs in Equisetum; though it is a 

 homosporous genus, some of its ancestral relatives were heterosporous, 

 and even now the gametophytes usually are dioecious, though arising 

 from approximately similar spores. However, the smaller and more 

 poorly nourished gametophytes usually bear male organs and the larger 

 gametophytes, female organs. Furthermore, the smaller spores are 

 likely to give rise to male gametophytes and the larger spores to female 

 gametophytes ; in the true ferns, however, there appears to be no re- 

 lation between the size of the spore and the sex of the gametophyte that 

 comes from it. Occasionally, the gametophytes are monoecious, the 

 female organs appearing last, as in ordinary ferns. In Marsilea, one of 

 the heterosporous pteridophytes, the development of fruiting organs 

 (sporocarps) may be incited by partial desiccation (as in the drying up 

 of a pond), by increased illumination, or by high temperature ; on the 

 other hand, fruiting may be retarded or prevented by placing the plants 

 under water in weak light, at low temperatures, or in crowded cultures. 

 In the microsporangia there are sixty-four primordia which develop 

 commonly into microspores, but of the sixty-four megaspore primordia, 

 only one develops, and that at the expense of the others nutritively. It 

 has been shown that by subjecting developing Marsilea sporocarps to 

 spraying by cold water, no megaspore primordia develop, but that struc- 

 tures resembling megaspores may be made to develop from microspore 

 primordia under optimum nutritive conditions, growth being at the ex- 

 pense of other primordia, as in the development of ordinary mega- 

 sporangia; sometimes such spores are sixteen times as large as ordinary 

 microspores. Thus it is suggested that heterospory may have arisen 

 from homospory through the influence of optimum nutrition on develop- 

 ing sporangia. 



Some ferns show interesting transformations of reproductive primordia into 

 vegetative organs; for example, in Osmunda and Botrychium there often are leafy 

 organs in the reproductive region, and in Onoclea the removal of the foliage leaf is 

 followed by the development of another foliage leaf from the primordium of the 

 reproductive shoot. 



Reproductive variations in the seed plants. — Vegetative and repro- 

 ductive periods. — In the seed plants it is convenient to distinguish as 

 the reproductive phase all of the complex phenomena, both sporophytic 

 and gametophytic, from the inception of the flower to the maturation 

 of the seed, contrasting this with the vegetative phase of the sporophyte. 

 As in the lower plants, a vegetative period always antedates the period of 



