154 JOHN H. SCHAFFNER Vol. XXII, No. 6 



treatment,* it becomes evident that identity of sex in duplicate 

 twins can ?wt be regarded as giving any conclusive evidence in 

 support of the hypothesis that sex is determined by Mendelian 

 factors. As shown in the examples of twins described above in 

 Numbers 9 and 12, the sex of twins has been completely reversed 

 and the reversal was identical for each twin of a pair. The 

 intermediate examples are even more striking than those with 

 pure sexual expression. Cases like the one described under 

 Number 6 must certainly be regarded as most remarkable in 

 view of the fact that the sex of the individual is so easily 

 changed. All these cases show that the nutritive balance or 

 whatever it is that determines the sexual state must act with 

 decided precision when individuals of like heredity develop 

 under like conditions. Although so far the writer has no evi- 

 dence that Arissema twins placed in different environments 

 would develop the opposite sexual states in any given season, 

 yet, in view of the fact that any ordinary individual can be 

 changed from season to season and that the pairs of twins have 

 actually reversed their sex to the opposite state in agreement 

 with a change in nutritive environment, it appears that such 

 must be the case. 



As shown by the examples listed above, certain pairs of 

 dichotomous twins show fluctuation of a considerable degree, 

 but so far this has been found mainly for size, folding of the 

 spathe, emergence from the ground, and the like. The varietal 

 characters, like coloring of the spadix and spathe, shape of the 

 sterile spadix and length of its stalk, shape of leaflets, color of 

 anthers and stigmas, etc., are remarkably alike in each pair and 

 deviate only in minute detail. 



* ScHAFFNER, JoHN H. Control of the Sexual State in Arisaema triphyllum 

 and Arisaema dracontium. Am. Jour. Bot. 9: 72-78. 1922. 



