212 



FUNDAMENTALS OF CYTOLOGY 



plant is of interest in genetical studies, for it is completely homozygous, 

 except for possible new mutations. It seems possible that a monoploid 

 race might be established in nature if the plant had efficient means of 

 vegetative reproduction, but no such case has been discovered. 



Among the many types of aneuploidy, in which one or more full 

 genomes are accompanied by one or more additional chromosomes not 

 constituting a full genome, the commonest and probably the most impor- 

 tant is the simple trisomic condition (2/i +1). Here the plant has all its 

 chromosomes in duplicate except one, which is present in triphcate. If 

 two members of the genome are in triplicate, the plant is said to be 

 doubly trisomic (2n +1 + 1), etc. Simple trisomic plants are of special 

 value, for they have normal fertility, transmit the extra chromosome to 



C 



)^ 



y 



to 



/\ > 



Fig. 158. — -Flower heads borne on monoploid 

 individuals of Crepis capillaris. Left: monoploid 

 head. Middle: diploid head, after chromosomal 

 doubling. Right: head with monoploid and diploid 

 sectors arising after local doubling. {After L. 

 H oiling shead.) 



Fig. 159. — Chromosomes in 

 pollen grain from trisomic Datura 

 stramonium. This grain carries 

 an extra 3.4 chromosome. (After 

 S. Satina, D. Bergner, and A. F. 

 Blakeslee.) 



some of their progeny, though onlj^ rarely from the pollen parent. The>' 

 thus serve to reveal the special functions of each chromosome of the 

 genome. This point has been explained on page 174, where the effect 

 upon genetical ratios was described. 



Since each chromosome of the genome differs from the others with 

 respect to the group of genes it carries, plants trisomic for different 

 chromosomes are expected to differ in visible characters. This expecta- 

 tion is met in some degree in the species investigated. In Datura stramo- 

 nium each of the 12 possible trisomic types can be distinguished from the 

 others (Figs. 159, 160). The same is true of the trisomic types, also 12 

 in number, in Nicotiana sylvestris. In Zea mays 9 of the 10 possible 

 trisomic types have been obtained, and most of these are distinguishable, 

 although the differences are less than might be expected. Moreover, 

 there are certain features, notably the reduced size of the plants and 

 seeds, that characterize all the maize trisomies in common. In addition to 

 these primary trisomic plants, in which the three chromosomes composing 



