314 ALLEN— SEX INHERITANCE IN SPH^ROCARPOS. 



apparent exceptions found by Douin were the result of an aberrant 

 behavior of the sex chromosomes ; but reasons have already been 

 given for doubting whether these exceptions were other than appar- 

 ent. In the examination of spore tetrads I have come across a few 

 instances of spores joined in twos rather than in fours ; and other 

 instances in which a group consisted of two spores of normal size 

 and one or two very small. Such conditions may result from an 

 irregular distribution of the chromosomes ; but they may equally 

 well or perhaps better be explainable by the death or accidental 

 injury or destruction of one or two spores after the completion of 

 the division of the mother cell. 



The existence of definite sex chromosomes having been estab- 

 lished for two species of Sphccrocarpos, it is to be expected that 

 similar bodies will be found in other plants. The most promising 

 organisms for such investigations are probably the other dioecious 

 bryophytes, especially those with a marked sexual dimorphism. 

 Next would come perhaps some of the dioecious algae. That pre- 

 vious searches for sex chromosomes in plants have been fruitless 

 has been largely because they dealt chiefly with dioecious seed plants. 

 Enough of these have now been examined to make it quite plain 

 that no visible chromosome difference is to be expected as between 

 the staminate and the pistillate individuals of any species. This 

 negative result is quite in harmony with what we now know regard- 

 ing Sphccrocarpos, because the distinction between staminate and 

 pistillate sporophytes is of a quite different sort from that between 

 male and female gametophytes ; and if anywhere in the seed plants 

 a chromosome difference is to be looked for exactly comparable 

 with that described in the present paper, it must be between the 

 micro- and the macrogametophytes. Evidence as to the possible 

 existence of such a difference is still meager; but such evidence as 

 does exist is, it must be admitted, of a negative character. 



In a previous paper (Allen, 1917a) I have referred to Hirase's 

 (1898) description of a cytological difference between the two an- 

 therozoids produced by the same microgametophyte of Ginkgo, 

 which might be imagined to be the basis of the dioecism of the sporo- 

 phytes of this species. This difference consists in the presence of a 



