446 MINNESOTA BOTANICAL STUDIES 



both of these interesting birches might be natural hybrids between 

 the common and well-marked species of the region. 



The fact that a number of birch species hybridize more or less 

 freely in nature and under cultivation, has been observed for a long 

 time, and several such hybrids have been described at different times. 

 On the grounds of possibility, therefore, there was no reason why 

 these forms might not be hybrids. But even though they might 

 suggest through their intermediate characters that they were hy- 

 brids, still to make a mere categorical assertion to that efifect without 

 any additional evidence, seemed somewhat unjustifiable. 



Several attempts to grow the seeds of both the hybrids have 

 been made, but up to the present only three seedlings from the last 

 sowing of B. Sandbergi have been obtained. These are yet too 

 young and small to offer any proof one way or the other so that the 

 hope of learning something through the segregation of parent strains 

 in the second generation has not been realized. Another evidence 

 of hybridity or of hybrid contamination in plant species has been 

 strongly emphasized in recent publications of Jeffrey.^ He has 

 found in his investigation of a large number of known hybrids that 

 the pollen always shows a large per cent of defective grains, and 

 further that in many families where crossings readily take place, 

 numerous forms occur whose parentage is not known, or is obscure, 

 but which have defective pollen grains and thus indicate hybridity. 



The pollen test was applied to all the birches involved in the 

 suspected crossings and the facts revealed are quite significant. It 

 was found that the pollen grains of the three common birches grow- 

 ing in the vicinity of the hybrids, namely Betula papyrifera, B. liitea 

 and B. pumila var. glandulifera, ran nearly 100 per cent good on all 

 the counts. This may readily be verified by an examination of the 

 photo-micrographs (Plate XLIX). On the other hand, the pollen 

 in the two suspected hybrid forms proved to have 25 to 40 per cent 

 defective grains. If, therefore, defective pollen in considerable 

 quantity is a reliable criterion of hybridity in plants, and Jeffrey's 

 extensive observations along this line appear to have established it 

 as a fact, then the assumption that the two birches in question are 

 natural hybrids is amply sustained. 



The intermediate character of the hybrids is made tolerably 



=* Jeffrey, K. C. Some I-'iinclamental Morphological Objections to the Mutation Theory 

 of De Vries. American Naturalist, 49:5. 1915. 



Spore Conditions in Hybrids and the Mutation Hypothesis of De Vries. Botatncai 

 Gazette, 58:322. 1914. 



