May 28, 1932] Nature 793

MAY 28, 1932] NATURE 793

The investigations are being continued with dif• male gametophyte ever produces archegonia. The ferent gases and different wall materials. fact that the female gametophyte may produce both R. M:. SIEVERT. types of sex organs indicates that the separation of Physical Laboratory of the sex tendencies in meiosis in Equisetum is not absolute. ' Radiumhemmet ', Hence it would probably be futile to look here for sex Stockholm, March 15. chromosomes. Nevertheless, it may be that nuclear differentiation is responsible for the fact that some 1 J. W: Broxon, Phys. Rev., 37, 1320; 1931. spores of Equisetum produce a male and some a female ' G. 1'. P. Tarrant, Proc. Roy. Soc., A, 135, 223; 1932. 3 E. G. Steinke and H. Schindler, Naturwiss., 20, 15; 1932. gametophyte. It needs to be definitely det ermined • R. M. Sievert, Acta Radwl., 12, 190 ; 1931. whether two spores of each tetrad are male and two ' G. Jaffe, Ann. Phys., 25, 257; 1908: and 42, 303; 1913. • E. Stahel, Strahlenthll1'., 31, 582 ; 1929. female in tendency. If this were so, the differentiation 1 A. H. Compton, R. D. Bennet, and J. C. Steams, Phys. Rev., 38, might have resulted from an endosporal chromosome 1565; 1931. m echanism for sex determination similar in its main features to that which is known in certain Liverworts and in certain dioocious flowering plants, but less fully Heterospory and the Angiosperms developed. FoR many years it has been generally assumed by It may be pointed out that such a nuclear differentia• botanists that in Angiosperms, as in the heterosporous tion of homospores would involve an evolutionary Pteridophytes, the megaspores are larger than the process of quite a different nature from that which has microspores. In a study of megaspore development in given rise to the differentiated megasporangia and CEnothera rubricalyx (Gates and Sheffield),l it was microsporangia of Selaginella. It is difficult to see incidentally discovered that this is not the case. Series how these two processes could have taken place even of measurements showed that the corresponding successively in the same series of organisms. Thus the absolute values were about 4930f1.a for the 'mega• heterospory in Selaginella could not have been super• spore' mother-cell and 12,630{).3 for the 'microspore' posed upon the condition found in Equisetum if the mother-cell at the end of meiosis. The need for com• latter is a case of morphological homospory but with parative measurements of the size of the spores in the the spores of each tetrad separated as to sex. On the pollen and ovules of various plants was pointed out, other hand, if the separation of spore types in Equi• and brief reference was made to the possible signifi• is only a partial one, environmentally controlled, cance of the above condition. 1t may have been brought about by the same kind of It has since been found that this subject has been evolutionary process as has produced the differentia• independently discussed at some length by Prof. tion of microspores and megaspores in Selaginella. R. B. Thomson,2 who has also made measurements of The differentiation, in the same cone, of micro• spore size in various Pteridophytes, Gymnosperms, and sporangia and m egasporangia containing spores which Angiosperms. He finds that in some Cycads, Conifers, develop their respective gametophytes, as in Sela• and Angiosperms the pollen spore is larger than the ginella, is a somatic differentiation which is completely seed spore, while in others the reverse is the case. independent of the meiotic divisions. It could not There is then at present no certainty that the Angio• have been preceded by spore differentiation of a type sperms were derived from heterosporous ancestors. in which the sex potentialities of the spores in each That the microspores are larger than the megaspores tetrad are separated. How the condition in Equisetum in CEnothera, Pontederia, and Typha, while they are was derived in turn from that in the homosporous smaller in Lilium, may be a local development in each leptosporangiate ferns, in which each spore produces case. It is to be expected that the morphological and a gametophyte bearing both male and female sex biological relations within the flower, such as size of organs, we do not know. ovule and length of style, will be factors in adjusting Thus it appears that the ancestral seed plants were the relative size of the spores which produce respect• derived from forms which developed heterangy, but ively the male and female gametophyte. Thus in not from homosporous forms with sex potentialities CEnothera the need for a long pollen tube necessitates separated in meiosis. Whether they had megaspores storage of food material in the pollen mother-cell. and microspores seems quite uncertain. Perhaps de• This is abundantly present in the cytoplasm and helps scendants of the modern eusporangiate ferns might to account for the large size of the cell. In the ovule, lay claim to represent such a group. Since, however, on the other hand, food storage occurs not in the ti;e relative of male and female spores in megaspore mother-cell but especially in the chalaza! different fam1hes of AngiOsperms can vary so widely, region of the nucellus. there seems no adequate reason at present for denying Thomson proposed the term heterangy for the that the group may have been derived from ancestors condition in seed plants where the m egaspore is re• which showed heterospory. The sporangium is so tained within the megasporangium (ovule), confining closely related to its enclosed spores that differentia• the term heterospory to those forms in which the tion of sporangia as an evolutionary process must be megaspores are discharged. Thus the ancestor from closely connected with differentiation of spores. This which seed-plants (Pteridosperms) arose might have means heterospory in the etymological sense, although been heterosporous in the sense in which the term is not necessarily in the sense in which that term is com• used in Selaginella and other Pteridophytes, or it monly used. might equally well have been homosporous but heter• A large and interesting field regarding the relative angious. Equisetum well represents an earlier con• sizes of the spores in Angiosperms, their evolutionary dition, homosporous, homangious, with the gameto• relationships, and their relations to various features in phytes developing externally to the spores. From the biology of the species, here awaits investigation. such a condition, seed plants showing heterangy and R. RUGGLES GATES- an intrasporal female gametophyte might have de• King's College, veloped. University of London, It is well known that although Equisetum is homo• April17. sporous, some of the spores produce a male gameto• phyte and some a female. The latter may, however, 1 Gates and Sheffield, " Megaspore Development in (Enothera sometimes develop antheridia as well as archegonia, rubricalyx ", Proc. Roy. Soc., B, 105, 499; 1929. ' Thomson, R . B., " Evolution of the Seed Habit in Plants " although it does not appear to be known whether the Trans. Roy. Soc. Oan., 21, 229 ; 1927. No. 3265, VoL. 129] © 1932 Nature Publishing Group