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Darling: Sex in dioecious plants 193 



tissue they find that one tendency may be dominant and the other 

 latent or recessive throughout the development of the gameto- 

 phyte ; or in other cases they find that both tendencies may be 

 potent so that both sex organs develop. 



In the Pteridophyta the tendency to dioeciousness, as noted by 

 PrantI and Duval-Jouve in the ferns and equisetums, may be ac- 

 counted for on the grounds that a complete dominancy of the two 

 tendencies does not take place at the time of spore formation, but 

 this dominancy arises at a later period depending directly or indi- . 

 rectly on external conditions. In the heterosporous Pteridophyta 

 the dominancy of one tendency occurs at some time previous to the 

 formation of the spores^ so that the two kinds are found in differ- 

 ent sporangia. The essential difference, therefore, between the 

 dioeciousness of the Bryophyta and of the Pteridophyta is that in 

 the former there is a dominancy and separation of the tendencies 

 in the formation of the spores, and in the latter there is no such 

 dominancy and separation attending spore formation. In the isos- 



■ 



porous ferns this dominancy occurs after the formation of the spores 

 and in the heterosporous forms before. This interpretation seems 

 to be in accord with the work of Morgan ('09) on the phyllox- 

 erans ; he finds that there are two kinds of eggs produced, a 

 large female-producing egg and a smaller male-producing one. 



These eggs develop parthenogenetically, each forming but one 



polar body. Both eggs previous to the formation of the polar 

 body contain the somatic number of chromosomes, twelve; after 

 the formation of this body, the female-producing one contains 

 twelve, the male-producing ten. Since the size of these two kinds 

 is evidently determined at another time than at the reduction divi- 

 sion, it may be cited as further evidence that a dominancy may 

 occur other than at the time of spore formation. 



The monoecious and hermaphrodite Spermatophyta present a 

 somewhat different condition. It seems very evident that the spo- 

 rophyte tissue is bisexual in character, but that one tendency may 

 dominate the other. Observations show that the willows, which 

 are usually dioecious, may occasionally develop both sex organs. 

 Another case is that of Melandrmm, as reported by Strasburger 

 ('00) ; when the pistillate flowers are attacked by Ustilago, the 

 primordia of the stamens, which otherwise do not develop, produce 



