1928] Wexelsen: Chromosome Numbers and Morplwlogy in Trifolium 371 



With both methods the results were completely negative ; a few 

 seeds obtained by either method proved to be due to selfing. Outside 

 of these there seemed to be no seed development at all. Below are 

 given the combinations which were tried and the number of flowers 

 crossed in each combination. All the crosses were made reciprocally 

 except in 4, 8, 10, 14, 15, and 18. 



In crosses 1, 2, 3, 5, 7, 9, and 11 the number of trials is large 

 enough to allow the statement that hybrids between these species are 

 not easily obtained. 



In pratense, re pens, hybridum, and virescens intraspecific crosses 

 were made and seeds easily obtained, so the negative results are not 

 due to faulty technique. T. fucatiim and T. virescens are two very 

 nearly related species or varieties of the same species which did not 

 seem to cross. The number of flowers crossed is not large, but when 

 crossing plants within virescens seeds were easily obtained. These 

 results do not, of course, allow the conclusion that hybrids cannot be 

 obtained between these species, but they suggest, in agreement with 

 the results of other investigators, that interspecific hybrids are difficult 

 to secure. 



In case of the crosses T. praiense x T. repens, and T. hyhridum, 

 respectively, it Avas attempted, using the method described by Martin 

 (1913), to study the behavior of the pollen of repens and hyhridum 

 on the stigma of pratense. Flowers of pratense were emasculated, 

 pollinated immediately, and the stigmas picked out for observation 

 after 18, 24, 48, and 72 hours. The stigmas were mounted on a slide 

 in aceto-carmine and a slight pressure was exerted on the coverglass 

 to flatten the stigma. The pollen both of repeiis and hyhridum 

 germinated readily on the stigma of pratense, but it was not found 

 possible to follow the pollen tube growth through the style by Martin 's 

 method. Nothing, therefore, was ascertained as to what happened to 

 the pollen tubes. It may be that the situation is the same as in self- 

 sterile species of TrifoUum in which, when selfed, the pollen will 

 germinate, but the pollen tube growth is too slow to reach the ovary. 



