218 BOTANY 



male and female elements took place. Ascogenous hyphse were 

 budded out in the usual way from the oogonium, and when their 

 tips bent over to initiate the young asci, simultaneous division of 

 the nuclei took place. In some of them the nuclei showed 7 

 chromosomes on the spindle at metaphase and 14 (7 to either 

 pole) at anaphase, i.e., the haploid number. In others, 24 chromo- 

 somes were counted at metaphase, and 20 could be counted on 

 their way to the poles, i.e., the diploid number. Thus the third 

 division might or might not be brachymeiotic. It is therefore 

 suggested that sometimes a fusion of male and female elements 

 takes place in the oogonium, and if such has occurred, then the 

 third division will be brachvmeiotic in nature. On the other 

 hand, fusion may not take place and the third division will then 

 be vegetative. Tandy has suggested that P. domesticum re- 

 presents a transitional form among the Ascomycetes. 



SEX-CHROMOSOMES 



That sex is controlled by special chromosomes has been known 

 for some times in the case of animals, and the matter has been 

 very extensively investigated by animal cytologists. In the case 

 of plants, however, their discovery is of very much more recent 

 date. 



In plants a large variety of sexual conditions are exhibited. 

 Sharp, following Blakeslee, distinguished the following : homo- 

 thallic plants, i.e., those manifesting the two sexes in the same 

 gametophyte or thallus, and heterothallic plants, those manifesting 

 sex in different gametophytes. Heterophytic plants are those in 

 which two kinds of spores, with male and female tendencies 

 respectively, are borne by separate sporophytes. Homophytic 

 plants are those in which a single sporophyte bears the two kinds 

 of spores, with male and female tendencies respectively, or spores 

 of one kind with a bisexual tendency. Thus the Bryophytes are 

 all homophytic, but they may be either homothallic or hetero- 

 thallic. Seed plants, on the contrary, are all heterothallic, but 

 they may be either homophytic or heterophytic. 



It was among the Bryophytes that evidence was first forthcoming 

 that sex was bound up with meiosis. The Marchals, for example, 



