100 INTROGRESSR^ HYBRIDIZATION 



has been observed. One can comparatively easily estimate 

 the probable outcome of crossing one inbred hne of maize 

 with another and then backcrossing one or two times to the 

 original line. It takes more experience to suggest what might 

 be the result of such an operation upon well-differentiated 

 species. When totally different genera (such as Zea and 

 Tripsacum) may be concerned, the possible effect of intro- 

 gression of either into the other is a research problem of no 

 mean dimensions. One may have studied genetics for a life- 

 time and still be totally unable to answer the question "What 

 would be the result of any one or two genes from Drosophila 

 if they were introduced into Zea Maysf^ 



In introgression, what often seems at first sight to be the 

 appearance of something totally new usually proves to be a 

 recombination that one had not had the wit to anticipate. 

 Hybridization ordinarily results not in the new, but in the 

 unexpected. For example, brilliant-colored stems and leaves 

 often appear when Tradescantia canaliculata suffers intro- 

 gression from Tradescantia suhaspera var. pilosa. Neither 

 of these species has conspicuous plant color. Careful ex- 

 amination, however, shows that T. suhaspera has a dull 

 purple pigment in the epidermis — so dull that it gives the 

 leaf and stem a general appearance of very dark green. T. 

 canaliculata has very little color in the epidermis, but what 

 there is has none of the dark purplish cast that characterizes 

 T. suhaspera. Introgression, therefore, brings some of the 

 basic genes for colored epidermis into T. canaliculata, and 

 when they operate there in the absence of the dark purple 

 modifiers they produce a brilliant effect superficially quite 

 different from anything in either species. 



In the studies of introgression between these species it was 

 not until after the artificial backcrosses had been made that 

 we began to suspect the origin of the suhaspera introgressants 

 in T. canaliculata. These two species are strikingly different : 

 T. canaliculata has a few long nodes, the uppermost of which 

 are usually the longest. T. pilosa has many short nodes, and 

 node length decreases progressively upwards. The intro- 



