CYTOLOGY AND TAXONOMY 



239 



closely similar in the morphology of their chromosomes, including the 

 position of their knobs. Southern Guatemalan teosinte has a genome 

 like that of maize except for knob position which is like that of gamagrass. 

 These features, together with chromosome l)ehavior in the crosses men- 

 tioned below, have an important bearing upon the question of the origin 

 of maize. A proposal to transfer the two species of Euchlaena to Zea has 

 already been mentioned (page 218). 



The use of chromosome morphology and number in the determination 

 of generic relationships is especially well illustrated in a recent study of 

 the Ranunculaceae (buttercup family). The conclusions reached in 

 this study are summarized in the lower phylogenetic chart reproduced in 



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Fig. 171. — Chromosome diagrams (idiograins) of maize (left), teosinte from southern 

 Guatemala (center), and gamagrass (right). Small circles, black spots, and shaded 

 regions represent kinetochores, knobs, and nucleolus organizers respectively. {After A. E. 

 Longley.) 



Fig. 172. The upper chart is a similar representation of the relationships 

 of the genera as conceived under the Engler system of classification. 

 The new scheme differs conspicuously from the old in associating more 

 closely the genera with small chromosomes. 



Chromosome Behavior in Crosses. — This type of evidence for relation- 

 ship has a special value. Chromosomes of two organisms may be similar 

 in form and visible structure and j^et be widely different in function. 

 When two organisms are crossed, however, the very fact that a hybrid 

 results shows that the two genomes brought together are sufficiently 

 alike to act iii harmony with each other and with the cytoplasm to permit 

 ontogenetic development. This is strong evidence of relationship, for 

 organisms that are obviously very distantly related do not produce 

 hybrids. 



A fiu'tlier critical test of relationship comes at the time of meiosis in 

 the hybrid. As already pointed out in previous chapters, synapsis 



