No. 577] MUTATION THEORY OF BE VRIES 



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cies of liverworts, belonging to the Marchantiales, Antho- 

 cerotales and Jungermanniales, both acrogynous and 

 anacrogynous have been examined with the general result 

 that the only sterile cells present in the capsule cavities 

 were the elaters. Infertile spores and hybridism both were 

 conspicuous by their absence in the forms studied. The 

 same statement mutatis mutandis holds for the true 

 mosses. Some indication of spore abortion was detected 

 in the extremely variable genus Sphagnum. It would 

 seem that natural hybrids exist to some extent in this 

 genus. Among the Pteridophyta both the Lycopsida and 

 Pteropsida were studied. None of the numerous Lycopsid 

 forms investigated showed signs of spore abortion or 

 hybridism. Among the Pteropsida, the only well-known 

 hybrids are found among what is probably the highest 

 family, the Polypodiaceae. There is a considerable litera- 

 ture upon hybrid ferns, in which references to spore abor- 

 tion as an accompanying feature are common. No evi- 

 dence of hybridism in the form of abortive spores was 

 found in examples of the Marattiaceae, OphioglossaceaB, 

 Osmundaceae, Gleicheniaceae, etc., were found, although a 

 large amount of material was examined. Among the 

 Gymnosperms, the Cycadales, Ginkgoales, Coniferales 

 and Gnetales were examined. The Coniferales yielded 

 only a single species of Abies, which showed evidence by 

 the presence of abortive pollen grains of hybrid origin. 

 The genus Pinus is very old and its species accordingly 

 very distinct. Not the slightest evidence of hybridization 

 was found here or in other numerous and widely distrib- 

 uted species of conifers, other than Abies mentioned 

 above. This does not of course preclude the discovery of 

 such conditions later. The writer has had the opportunity 

 of examining the spores of a number of fossil forms from 

 the Paleozoic and Mesozoic, still contained within the 

 sporangia, and in no case were abortive spores recognized. 

 The general conclusion can be drawn from the forms just 

 considered that hybridism is rare among them and that 



