No. 577] MUTATION THEORY OF BE VRIES 



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detection of hybridization. Physiological sterility is fre- 

 quently due to entirely different causes than genetical lack 

 of harmony, as for example in the horseradish or the 

 potato {Solarium). In the former it has been found pos- 

 sible to bring about the formation of fertile seed by simply 

 girdling the top of the subterranean storage region of the 

 plant, so as to prevent the undue descent of assimilates. 

 The common white lily, Lilium candidum, presents a 

 similar condition, for here the setting of seed takes place 

 only when the leafy flowering axis is severed from its 

 bulb and kept in water. So far as I am aware, there have 

 been no experiments as to the result of severing the con- 

 tinuity of the phloem (girdling), in relation to the restora- 

 tion of seed production in the potato. The common yellow 

 day lily (Hemerocallis) possibly presents a case similar 

 to that of Lilium candidum, for it does not ordinarily set 

 seed, although in all the examples I have examined the 

 pollen was morphologically perfect. I have not yet been 

 able to secure flowers of any pure species of Fuchsia, a 

 genus which flourishes mostly in the remoter parts of 

 South America and in the New Zealand islands. The 

 cultivation of Fuchsias, although once very popular, has 

 now gone out of vogue and it is consequently difficult to 

 secure specimens of the species. As has been pointed out 

 the commonly cultivated Fuchsias are of hybrid origin. 



We may now turn our attention to a very puzzling genus 

 of the Onagraceae, namely Epilobium. This genus has 

 been a great riddle to systematists and the determination 

 of species has been extremely difficult on account of their 

 extreme variability. In European systematic works, this 

 high degree of variability is recognized clearly to be 

 largely due to hybridization and in such a standard work 

 as the "Naturliche Pflanzenfamilien " of Engler and 

 Prantl, the statement is definitely made that the various 

 species of Epilobium frequently and commonly hybridize 

 with one another in nature. Let us consider in this con- 

 nection the northern hemisphere cosmopolitan species, 

 known as Epilobium amgustifolium, the willow herb or 



