E. J. Collins 141 



heriimphnidito. Tho ditticulty is recogniHeti by Kl. and Km. Miiix:hiil, 

 but in alloviiition of this objection th» y mh^'^^'mI that jiinon^ thi- male 

 plants may have b<»en many in which the develnpmrnt of J or^anM may 

 have lH*en suppre88e<l. Th»' a|H)s|H>rous pHwhirt^ of th** dioirouM forms 

 B, caespiticium, li. avgenteum, H. rapillarc, Ji. fall(Lr and M. htnumn 

 were without exception sterile. A|)oH]M)rou.s |)r<Kiiiets of moiioicotis Innns 

 were fertile, pnxlucing diploid gametes. 'I'hcHc united in frrtiliHiition, 

 and by further a|HW|)orou8 development from the s|)oroj(onium tetraploid 

 gametophyt<»s wei*e forme<l. 



In supjx>rt t>f the theory of sex segn'gation at HjX)rogeneHiH thr 

 reconled sex behavi»>ur of Sphaerocarpm terrestins is often (pioted. Ah 

 is well known, the s|H)res of this HejMitic are shed in the orif^inal tetrad 

 groups and aeconling to V. Douin, two ^ and two J plants result from 

 the development of the spores of each tetrad. More precisely, of 81 s{x>re 

 groups examined 64 showed two J" and two J plants, 13 germinated 

 inn>erfectly and 4 showed results not in agreement with ex|Xictation. 



Recently Allen' hjus rejwrted a chromosome ditierence correlated with 

 sex diffei-ences in Spha^rocarptis Donelli, and his limited observations 

 indicate' that of the spores of each tetrad, two give J* and two give 

 J plants as in X terrestins. At the reduction division two of the four 

 spores each receive a large chromosome and these develop female plants, 

 whilst the other two receive a small chromosome each and give rise to 

 male plants. 



It will be seen that the theory emphasises the dual nature of the 

 sporophyte generation, in that it carries both sex determinants. The 

 determinants however biing about no expression of sex in the sporo- 

 gonium, but assuming that dioicous sex plants were foreshadowed by 

 two types of spores, a differentiation of sporophytic tissues in conformity 

 with the change might ensue. Thus changes would be initiated which 

 Would give the s{X)rophyte the appearance of "sex monacism," and 

 inhibition of the differentiation of one or other of the " sex organs ' 

 would letwl to the dioicous spor<>phyte. Should the relative importance 

 of the two generations in the life cycle become reversed, the parasitism 

 of the -sexed plants would bring about a still greater differentiation ot 

 ti.ssues of the sporophyte in accordance with the physiological need. 

 Needless to .siiy the changes would lead to the impression of "six 

 charactere " ujx)n the sporophyte. 



Scheme I is suggested as an expression of the theory for dioicous 

 fonns, whilst Scheme II would apply to monoicous types. In the latter 

 » Science, N.S. Vol. xlvi. 1917. p. 4G0. 



10—2 



