214 



FUNDAMENTALS OF CYTOLOGY 



the trisome are all alike, Datura has been found to have a number of 

 secondary trisomic types, in which the extra chromosome consists of two 

 similar arms as the result of some form of aberration (Fig. 161). Since 

 there are 12 chromosomes in the genome, 24 secondaries should be pos- 

 sible if each of the two arms of every chromosome were to give rise to such 

 a chromosome with similar arms. More than half of these have actually 

 been found in Datura. The same phenomenon is also known to occur in 

 maize, although not so many of the secondaries have yet been discovered. 

 The special characters of these plants show that each half of each chromo- 

 some has its own distinctive effect upon the many reactions involved in 

 development, which is what one should infer from the fact that they carry 

 different groups of genes. 



Monosomic plants, with one less than the normal diploid chromosome 

 number {2n — 1), are rarely encountered, evidently because of the serious 



Fig. 161. — Diagram of the production of chi'omosomes with two similar arms ("secondary" 

 chromosomes) by the misdivision of the kinetochore. 



unbalance in the chromosome complement caused by the loss of one mem- 

 ber. Autopolyploids, however, are frequently found with a chromosome 

 missing as a result of irregular disjunction of multivalents; here the loss of 

 a single member causes less unbalance. In tobacco plants, which are 

 allotetraploid (page 222), nearly all the 24 possible types lacking one 

 chromosome have been found. 



Other aneuploid types \\ith varying numbers of additional chromo- 

 somes are found in considerable variety, especially in hybrids between 

 members of a polyploid series. The number of extra chromosomes in 

 some of these is rather high. Ordinarily fertility and vigor among aneu- 

 ploids approach the normal as the composition of the complement 

 approaches that in a plant with complete genomes. 



Significance of Autoheteroploidy in Nature.^ — After observing the 

 alterations in characters following the spontaneous or induced doubling of 

 the chromosome number, and after considering the prevalence of poly- 

 ploidy among angiosperms in nature as revealed by lists of reported 

 chromosome numbers, the conclusion that doubling and character 

 differentiation have been causally related in the natural evolution of these 



