82 Journal of the Mitchell Society [September 



ment. ^^'^len mature the diameter of the tetraspore mother-cell is 

 nearly twice that of the egg. 



The relatively small number of plants which reached maturity in 

 these cultures as compared with the much greater numbers secured in 

 subsequent repetitions is believed to be due to lack of proper illumi- 

 nation resulting from the fact that the shells were so attached to their 

 supports as to present their edges to the incident rays of light. Con- 

 sequently, relatively few plants were favorably situated as regards 

 sunlight. Especially is this worthy of consideration when one recalls 

 that rays of light are refracted towards the normal on passing from 

 air into water. Probably, also, some cultures were located on the 

 north sides of their supports, and hence received no direct rays at all, 

 as this detail had not then occurred to the writer. These ideas to- 

 gether with the further probability that the optimum depth which 

 also appears to lie within narrow limits was not in all cases secured 

 would, it would seem, account for the fact that no plants matured on 

 5 of the 16 shells, all of which were covered with sporelings when 

 planted. 



From the above seven successful cultures, however, it may be stated 

 with certainty that tetraspores produce only sexual plants, since there 

 were no tetrasporic individuals whatever produced. 



Attention should, perhaps, also be called to the fact that each of 

 these successful cultures was derived from a single tetrasporic plant, 

 and that in every case both males and females were produced and in 

 approximately equal numbers with the exception of culture 3b, which 

 gave only 2 mature plants and these females. The small number in 

 this and the other cases, it is believed, sufficiently accounts for the 

 slight disparities. Comparison with the results obtained in 1914 

 and 1915, shown in Tables 4 and 8, where the numbers are larger, 

 adds probability to this conclusion. These results indicate that sex 

 is predetermined, probal)ly when the spores are formed, and that one- 

 half the spores bear the determiner for malenes^s and the other half 

 for femaleness. This conclusion is rendered more probable when we 

 recall that the spores producing male and female plants in equal 

 numbers were derived from a single tetrasporic })lant and grew upon 



