414 



SCIENCE 



[N. S. Vol. LIV. No. 1400 



mosomes, and the Vulgare group, consisting 

 of T. vulgare and T. compactum, has 21 hap- 

 loid chromosomes. 



A study of the sterility relationships of spe- 

 cies crosses has already been completed and is 

 of considerable interest in connection with the 

 chromosome number. Einkorn with 7 chro- 

 mosomes crossed with members of the Emmer 

 group with 14 chromosomes or with members 

 of the Vulgare group with 21 chromosomes, 

 results in almost totally sterile F^ plants. 

 Members of the Emmer group crossed with 

 members of the Vulgare group result in only 

 partially sterile F^ individuals. Species within 

 each group are inter-fertile. 



A review of the wheat crosses made reveals 

 the fact that practically the only hybrids of 

 economic importance are crosses within the 

 Vulgare group. The Emmer group possesses 

 many valuable characters such as drouth and 

 rust resistance, and certain varieties are heavy 

 yielders under some conditions. Many crosses 

 have been made between the members of the 

 Emmer and Vulgare groups, but very few, if 

 any, of the segregates have combined the de- 

 sirable characters of both parents. It is pos- 

 sible that all Fj gametes containing approxi- 

 mately half Vulgare chromosomes and half 

 Emmer chromosomes are sterile and only 

 gametes containing , nearly all Vulgare or 

 nearly all Emmer chromosomes survive. East^ 

 has suggested that such behavior may occur 

 in certain Nicotiana hybrids which are par- 

 tially sterile. Work now in progress makes 

 this conclusion rather doubtful for wheat hy- 

 brids. An analysis of six characters involv- 

 ing 80 F„ individuals of a cross of T. durum 

 X T. vulgare does not indicate that there is 

 greater sterility in the intermediates than in 

 seregates resembling the parents. 



There is a rather striking correlation be- 

 tween chromosome number and adaptability 

 among the species of wheat. Einkorn with 

 only 7 haploid chromosomes is of practically 

 no economic value. In the United States it 

 is grown only for experimental purposes. In 

 the Emmer group with 14 haploid chromo- 

 somes, T. durum is the only species grown 



Troc. Amer. Phil. Soc, Vol. 54, 1915. 



commercially in this country. The durum 

 wheats are for the most part limited to the 

 plains of the Dakotas and Montana. The Vul- 

 gare group with 21 chromosomes is in general 

 the most adaptable of the three groups of 

 wheat. It is grown in practically all parts 

 of the United States from Maine to Califor- 

 nia, in humid sections of the central states, 

 and on the semiarid plains of the western 

 states. There is certainly a high degree of 

 correlation between chromosome number and 

 adaptability of the species of wheat, but it 

 would be difficult to prove that adaptability 

 is due primarily to diifer^nces in chromosome 

 number. 



The fact that the chromosomes are in 

 multiples of 7 suggests that the species hav- 

 ing 14 and 21 chromosomes are the result of 

 reduplication of the 7 chromosomes of Ein- 

 korn or wild wheat. There is some evidence 

 that the larger chromosome numbers are due 

 to reduplication rather than fragmentation. 

 If we assume that the size of a given cell 

 is dependent on the chromosome content, 

 the relationship of the three groups of wheat 

 species becomes clearer. We have found that 

 the volume of the mature pollen gi-ains, meas- 

 ured in thousands of cubic microns, is about 

 72 for Einkorn, 94 for the Emmer group, and 

 114 for the Vulgare group. The differences 

 in chromosome numbers of the three groups 

 of species are closely associated with corre- 

 sponding differences in size of pollen grains. 



In the reduction divisions of the P, 

 hybrids of crosses between members of the 

 Emmer and Vulgare groups we find addition- 

 al evidence that the larger chromosome 

 numbers are the result of reduplication rather 

 than fragmentation. When the chromosomes 

 pair for the reduction division we find only 

 14 pairs of chromosomes and 7 single chromo- 

 somes on the heterotypic plate. The members 

 of the paired chromosomes separate and pass 

 to the poles leaving the 7 single chromosomes 

 on the equatorial plate. These single chro- 

 mosomes ultimately divide and pass to the 

 poles. If the 21 chromosomes of the Vulgare 

 group are the result of fragmentation we 

 should ex]3ect that homologous segments 



