■iSi^ SUMMARY OF CURRENT RESEARCHES RELATING TO 



of variation for progressive variation is very low, probably between one 

 in one million and one in one thousand. Regressive variation is much 

 more common. 10. The variations are all discrete or discontinuous, 

 not proceeding by imperceptible differences. 11. The progressive 

 variations in their differences actually parallel the differences existing 

 between many wild species of ferns and flowering plants. - 12. The 

 main difference between these variations and those shown by wild forms 

 lies in the fact that these horticultural forms do not possess adaptability 

 to natural conditions. In most cases, with the variation has come 

 decreased vigour of growth. 18. The causes of these variations are 

 undetermined ; they seem to be the response to internal stimulation. 

 Improved cultural conditions do not appear to cause the variations, but 

 are the means of preserving forms which, under wild conditions, would 

 be eliminated by natural selection. Several forms are figured, and a 

 genealogical chart is provided. 



Polypodium Species of Tropical America.* — W. R. Maxon publishes 

 a further contribution to his study of tropical American ferns. Three 

 groups of Polypodium have been widely misunderstood in the past, 

 namely, P. trichomanoides, P. furfuraceum, P. sfiuamatum, and their 

 respective groups of allies. He gives a synopsis of each group, with a 

 key to the species, photographs of several type-plants, descriptions of 

 eleven new species, and critical notes on the rest. He describes also 

 five other new species of Polypodium, and supplies critical notes on 

 species of Notholdena about which there has been much confusion. 



Bryophy t a. 

 (By A. Gepp.) 



Gemma-formation in Marchantia.f — J. Doposcheg-Uhlar writes 

 on the formation of external and internal gemmiferous cupules on the 

 male receptacles of Marchantia. He says that in the Marchantia 

 geminata of Java the antheridial receptacle may pass from the generative 

 condition to the vegetative ; single lobes (rays) cease to form antheridia, 

 continue a thallus-like growth at the apex, and produce gemmiferous 

 cupules. These cupules may arise in a normal manner superficially at 

 the vegetative apex. Gemmae are also found inside the transformed 

 thallose portion of the lobe (ray), as well as in the old unchanged 

 portion, in old empty antheridial cavities, and also in stomata. The 

 origin of these gemmte lies either at the base of these hollow spaces or 

 below them. Above these inner gemmiferous cavities are formed 

 " stif te," which probably by separating and opening become structures 



* Contrib. U.S. National Herb. Wasbingtou, xvii. (1916) pp. 541-608 (12 pig.), 

 t Flora, cviii. (1915) pp. 261-70 (14 figs.). See also Bot. Centralbl., cxxxi. 

 (1916) pp. 592-3. 



