274 The Phenomena of Morphogenesis 



McClintock (1929) reported a case in maize where the microsporocytes 

 had 19 chromosomes but the root tips 20, suggesting that a chromosome 

 had been lost during the development of the upper part of the plant. 



A number of instances are known in hybrid plants where twin stripes 

 or spots occur, differing from each other and from the background 

 color. These have been interpreted as the result of crossing over in so- 

 matic cells. Thus in Phaseolus the Fj of a cross between plants with violet 

 and with lilac flowers produced a form with light violet flowers. In one 

 of these there were two stripes, side by side, one of them violet and one 

 lilac (Prakken, 1938). A sectorial chimera presumably due to such 

 vegetative segregation was reported for a pear fruit by Gardner, Crist, 

 and Gibson ( 1933 ) . Twin spots, also apparently caused by somatic 

 crossing-over or chromosome translocation, are frequent in maize peri- 

 carp (Jones, 1938). 



Huskins and others (Huskins, 1948; Huskins and Cheng, 1950) re- 

 ported numerous instances where, as the result of low temperature or of 

 various chemical treatments, somatic mitoses occur in which the number 

 of chromosomes is reduced, as it is in meiosis. Wilson and Cheng 

 (1949) found that in such cases members of homologous pairs separated 

 much oftener than they would have done by chance, indicating a true 

 genetic segregation in the body cells of a heterozygous plant. 



In all these instances of genetic alteration in a few cells, the difference 

 between these cells and the normal type is usually not very great, and 

 there is less to be learned morphogenetically than in grafts and chimeras. 

 Whenever genetically different tissues from any source are present to- 

 gether in the same individual, however, their coexistence in a single 

 whole is evidence of the organizing capacity of living stuff. 



