1918] Alteris^atiox and Parthenogenesis in Padina 83 



the same shell. It is difficult to think that the conditions here could 

 he sufficiently diti'erent to have a determining eifect upon sex. The 

 view above expressed is of course in line with modern views of sex 

 determination. 



There remains to be discussed, perhaps, the fact that in Tables 4 

 and 8, which give the results of tetraspore cultures made in 1914 and 

 1915 respectively, tetrasporic plants occur — 2 in 1014 and 20 in 1915. 

 By reason of their location, now to be discussed, the chances for con- 

 tamination by fertilized eggs floating to and settling upon the shells 

 after they had been planted, were greater in 1914 than in 1912, and 

 still greater in 1915. Since, as has already been pointed out, cultures 

 were unsuccessful except where Padina normally occurred, it was 

 absolutely necessary to locate them in the vicinity of other plants. 

 In 1912 (Table 1) the shells were placed wnth their surfaces perpen- 

 dicular to the surface of the water, which of course made the likeli- 

 hood of chance spores lodging upon them very slight indeed, even 

 though an abundance of plants bearing spores was nearby. 



In 1914 (Table 4) the shells were attached to flat stones. The 

 stones were then sunk some distance away from plants but in the 

 general region of Padina growth and in such position that the sur- 

 faces of the shells bearing the cultures were parallel to the surface of 

 the water. The better illumination would probably account for the 

 greater number of plants reaching maturity per culture than in the 

 former case. In 1915 (Table 8) the cultures were attached to stones 

 as before and then placed in among growing Padina, merely removing 

 the plants touching the cultures and those innnediately around, their 

 abundance rendering it impracticable to do more on accoiuit of the 

 physical labor involved. The chances of contamination being thus 

 vastly greater, it is not surprising to iind n hirger number of intruders. 

 The facts above recited sufficiently explain, it is believed, the absence 

 of tetrasporic individuals in 1912, the ])resence of 2 in 1914, and 2(i 

 in 1915. 



Perhai)s it shoidd be ii(lde<l here th:it tliese experiments were at- 

 tempted also in l!>i;'), but the snpi)orts to wiiieh tliey were attaeheti 

 were undermined and washed awav hv a severe -im m which -wept the 



