406 SECTIONAL TRANSACTIONS.— K. 



zygote rapidly divides and the plasmodium soon enters the wood. After emergence, 

 portions of the protoplasm degenerate and give rise to the pseudo-capillitium and wall 

 of the sporangium. In the remaining portions the nuclei undergo two successive 

 divisions before spore formation, constituting a typical meiosis. 



Eeticularia is regarded as a primitive type among the Mycetozoa. 



9. Dr. E. J. Collins. — Sex Distribution and Inheritance in Sileyie mutans 

 in Relation to the Sex Problem in Plants. 



In Silene mutans — a reputed trioecious species — four types of plants have appeared 

 during this investigation, carried on since 1917. 



(a) Hermaphrodite. 



(6) Those bearing hermaphrodite and female flowers together with flowers with 

 a reduced number of stamens, frequently with a bias. 



(CI) Seeding females. 



(C2) Non-seeding females. 



Non-seeding females come in practically all progenies whatever the parentage. 

 Investigation has shown that the embryo-sac is not organised in this type. 



The predominantly female type self-pollinated gives a progeny in which maleness 

 is reduced or entirely absent, irrespective of the sex form of flower made to bear the 

 seed. With pollen from a plant constant in its hermaphroditic condition, malenesa 

 in the progeny is increased. 



No purely male form has appeared at any time. 



10. Discussion on Sex-determination in Plants. 



(a) Prof. Dame Helen Gwynne-Vaughan, D.B.E. 



In several dioecious species of Lychnis, in Elodea and in Rumex, the sporophyte 

 of the male has been shown to possess X and Y chromosomes, and that of the female 

 XX ; the case for sex-determination here seems to be strictly comparable with that 

 in animals. While there is only one kind of embryo-sac, there are pollen grains of 

 two types, male-producing and female-producing. A possible comparable condition 

 is found in Mucor. 



In monoecious sporophytes, on the other hand, there is at present no evidence 

 of the part played by the chromosomes in the development of sex, or whether 

 monoecious and dioecious species of the same genus differ essentially in this respect. 



Among homosporous plants with an asexual sporophyte, the gametophyte may 

 be monoecious or dioecious. In Sphmrocarpus the nuclei of archegoniate plants 

 possess a large X chromosome, while in antheridial plants the nuclei contain a smaller 

 Y element ; there is evidence that both occur in the nuclei of the sporophyte and that 

 X and Y are synaptic mates. A similar condition is indicated in dioecious mosses 

 and in Phycomyces. Comparison with the male plant of Lychnis is of interest. 



In certain Basidiomycetes the basidia bear four spores, which have respectively 

 the constitution AB, Ab, aB, and ab ; a sporophyte is formed only when fusion 

 brings about the combination AaBb, and the existence of four sexes, and even of 

 larger numbers, has been inferred. Such Basidiomycetes lack gametangia and are 

 regarded as pseudapogamous forms in which all trace of sex in the ordinary sense has. 

 disappeared. 



(6) Dr. Heslop Harrison. 



Whilst it is true that in most of the Salicacese, as far as examined, sex chromosomes 

 have been discovered, and that the conditions are much the same as in animals, the 

 mere possession of such chromosomes does not explain the whole of the facts. In 

 the polyploid SaUces, for example, the preponderance of female plants is very great. 

 This seems the more remarkable when it is realised that the possession of a simple XY 

 group characterises the males. Hence differential growth of pollen- tubes from female- 

 producing grains, and similar phenomena, must play a part. 



Again, in spite of the fact that in the Caprese group of Salices the tetraploid species 

 S. aurita and 8. cinerea possess an XY pair in the male, up to the present a similar 

 pair has evaded detection in S. Caprea, presumably the more primitive form from which 

 S. aurita and S. cinerea have been derived. This would suggest that the S. Caprea 

 is in an indifierent condition, and would account for the marked oscillation in sex 



