478 The Origin of Polyploids 



tomato adventitious shoots arise spontaneously from wounds 

 covered with lanolin, they are not formed when the lanolin con- 

 tains indole-3-acetic acid. 



Chromosomal Chimeras 



Some adventitious buds that arise from callus tissue appear to 

 have arisen from a region on the surface of the wound that has 

 contained both diploid and tetraploid cells, for some shoots ap- 

 pear to be of mixed nature. Jorgensen found a nightshade plant 

 with a diploid core covered with a tetraploid skin and a tomato 

 shoot that was also composed of both 2n and 4n tissue. A plant 

 that consists of genotypically diverse tissue is a chimera, and if 

 the different kinds of tissue differ with respect to chromosome 

 number, the plant is a chromosomal chimera. 



Chromosomal chimeras have been produced in Datura by 

 Blakeslee and his co-workers by treating seeds of diploids with 

 colchicine. After the treatment, the germinating seeds were 

 found to be mixtures of 2n and 4?i tissue. In some, periclinal 

 chromosome chimeras were produced in which the cells of the 

 inner part of the branches was 2n and those of the outer parts 

 were 4n. The valuable feature of this work is that in the devel- 

 oping shoot the 2n and 4n cells can be distinguished readily by 

 their size. Other chimeras contained Sn tissue, which was also 

 distinguishable from the diploid and tetraploid cells. By study- 

 ing various periclinal chimeras which differed with respect to 

 .the chromosomal constitution of the outer and inner regions, it 

 was possible to identify the three independent germ layers of 

 the shoot and floral apices and to determine the contribution of 

 each germ layer to the formation of each organ. This ability to 

 distinguish the products of various germ layers has been of great 

 value in ontogenetic studies. Chromosomal chimeras, both peri- 

 clinal and sectorial, have been reported by Dermen in cranberry. 

 In this plant also they are valuable for studies of histogenesis of 

 primary tissues and the ontogeny of various plant organs. 



QUESTIONS AND PROBLEMS 



1. In a certain area plants of Tradescantia paludosa (n = 6) and 

 hybrids (n = 12) between T. canaliculata and T. hirsutiflora were grow- 

 ing near one another. Tetraploid hybrids were found with characters 



