236 



CYTOLOGY 



be lost, or additional chromosomes 

 may be added. Of greatest significance 

 are the cases where the ordinary or 

 diploid chromosome number becomes 

 doubled, producing what is known as 

 a polyploid. It has been found that 

 polyploidy has been a major factor in 

 plant evolution. A polyploid does not 

 produce fertile progeny easily when 

 crossed with its parental diploid, be- 

 cause the progeny will have an odd 

 number of chromosome sets, and will 

 fail to a large extent to produce sperms 

 or eggs with complete sets of chromo- 

 somes. A pol}ploid, therefore, has a 

 degree of reproductive isolation which 

 enables it to carry on an independent 

 existence and to evolve along its own 

 line. A very large number of species of 

 plants show evidence of chromosome 

 doubling in their evolution— in fact, 

 doubling has occurred more than once 

 in the ancestry of many plants. A curi- 

 ous fact has been discovered in the 

 course of studies of polyploidy: if dou- 

 bling occurs in a plant whose parents 

 were very closely related, the resultant 

 polyploid is likely to be more sterile 

 and less able to maintain itself than 

 if its parents had been very unrelated. 

 This fact, the reason for which we 

 cannot go into here, is of great prac- 

 tical value. It is often advantageous 

 to combine the desirable traits of dif- 

 ferent varieties or species, but when 

 one crosses these, the resultant hybrid 

 proves to be sterile— the chromosomes 

 of the two species are too unlike to 

 pair and separate properly during the 

 process of germ cell formation. Such 

 a cross would not be able to propagate 

 itself, therefore, were it not for the 

 possibility of pohploidy. If one induces 

 the chromosomes to double in such a 

 hybrid (and this can be done easily 

 with the use of colchicine or other 

 chemical), the resultant polyploid, 

 which will still possess the desired 

 combination of characters, will be 

 found to be perfectly fertile and ca- 



pable of passing its desirable combina- 

 tion of genes to succeeding genera- 

 tions. The wider the cross and the 

 more sterile the hybrid, the more likeli- 

 hood that a polvploid derived from it 

 by chromosome doubling will be per- 

 fectly fertile, and vice versa. 



In the course of evolution, chromo- 

 somes are apt to suffer various major 

 or minor alterations in structure. They 

 may lose small segments, or certain 

 segments may become duplicated. 

 They may experience inversions, or 

 they may exchange segments with 

 other chromosomes. As a result, chro- 

 mosomes in races which were origi- 

 nally derived from a common ancestor 

 may become ven' different in structure, 

 so different that if they are brought 

 into the same plant by appropriate 

 crossing they can no longer associate 

 in pairs as corresponding chromosomes 

 are supposed to do at the time when 

 germ cells are produced. Cytologists 

 can study the behavior of chromosomes 

 in hybrids and from their behavior at 

 this stage judge as to their similarity of 

 structure and hence their evolutionary 

 relationship. Bv techniques of this sort, 

 much has been learned about the an- 

 cestry of plants important to our econ- 

 om}', such as wheat and oats. 



Cytology demonstrates in unusual 

 degree the essential oneness of living 

 nature. The average person is apt to be 

 impressed more with the diversity than 

 with the unity of life. Plants seem so 

 different from animals. Bacteria seem 

 so different from either plants or ani- 

 mals. The millions of different species 

 of animals and plants, and the range 

 from amoeba to man, from the micro- 

 scopic alga to the sequoia tree, are 

 evidence of the ability of protoplasm 

 to take on myriad forms, to adopt mul- 

 titudinous variations of structure with- 

 out losing that structural key which 

 makes it alive. Truly the diversity of 

 living material is a profoundly im- 

 pressive fact. And yet, it is also an 



