HETEROGAMY. 



193 



Change from the monoecioiiB to the dioecious condition. This 

 is of less frequent occurence than might have been 

 anticipated. In the * Gardeners' Chronicle,' 1847, 

 pp. 541 and 558, several instances are noted of walnut 

 trees bearing female flowers to the exclusion of males. 

 The mulberry tree has also been noticed to produce 

 female blossoms only, while in other plants male 

 flowers only are developed. 



It seems probable that the age of the plant may 

 have something to do with this production of flowers 

 of one sex to the exclusion of the other. 



Change from the dioecious to the monoecious condition. Andro- 

 gynism. This is of far more common occurrence than 

 the preceding. 



Fio. 101. MonceciouB inflorescence of Hop. 



In the hop {Humulus Lupulus), when monoecious, 

 the female catkins are usually borne on the ends of 

 the branches as shown in the cut (fig. 101 ), and a similar 



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