SPECIAL TECHNIQUES 101 



gressants of suhaspera tend to have brilliant stems and leaves 

 and a much higher node nimaber than ordinary canaliculata. 

 Though their nodes are somewhat shorter than in the latter, 

 the extra number more than compensates, and the intro- 

 gressants are frequently twice as tall as their unmongrehzed 

 sisters. These tallish, bright-stemmed canaliculata^^ super- 

 ficially do not look at all like T. suhaspera pilosa. It is only 

 when careful studies are made of leaf shape, inflorescence 

 characters, and pubescence that one finds that the whole 

 complex in a greatly diluted form is tending to stay together 

 in these peculiar variants. 



After a few examples of introgression have been studied it 

 is much easier to recognize introgression in other genera and 

 in other families. With active introgression, the segregation 

 of whole chromosomes and of chromosome segments pro- 

 duces an overall effect on the variability of the population 

 which, though difficult to describe, is almost unmistakable 

 to those who have learned what it signifies. In such a pop- 

 ulation several different characters will be varying and re- 

 combining to a degree so far beyond what happens without 

 introgression that it is of another order of magnitude. Those 

 who have pioneered in the analysis of introgression are some- 

 times accused of ' 'seeing hybrids under every bush." The 

 truth of the matter is that, in certain groups of plants and 

 animals, the results of hybridization are more widespread 

 than had previously been suspected by most biologists and 

 that the morphological effects of hybridization upon popula- 

 tion variabihty are of a peculiar sort. With a little practice 

 these peculiarities can often be recognized, even in famiUes 

 r^ of plants and in floras with which the investigator is un- 

 famiUar. By methods like those outlined above, it is pos- 

 sible to apply a series of critical tests to such a varying popu- 

 lation and make valid estimates of introgression. 



