No. 577] 



MUTATION THEORY OF BE FRIES 



11 



partially included in the plane of section, it is quite clear, 

 that like those of Fuchsia, figured above, they have pro- 

 jecting germination pores, but unlike the Fuchsia of our 

 illustration, all the pollen grains of Epilobium (Chamce- 

 nerion) angusti folium are perfectly developed. I have 

 examined the pollen of the species under discussion from 

 widely separate geographical regions and under different 

 conditions of growth and season, with the uniform result, 



that the pollen is perfect and invariable in any important 

 respect. E. angustifolmm is a species which apparently 

 is not known to hybridize with other species and indeed it 

 is not easy to see how it could cross with those having 

 their pollen grains in tetrads. The perfection of the 

 pollen in view of this condition appears particularly sig- 

 nificant. The failure of E. angustifolium to hybridize in 

 nature with other species of the genus is doubtless due to 

 the fact that it is morphologically very distinct from these 

 and would in all probability produce, if artificially crossed, 

 only sterile hybrids. 



We may now turn by way of comparison to a species 

 of Epilobium of the ordinary type. Fig. 4 illustrates 



