542 Cytogenetics and Evolution 



These examples of plastid inheritance could be increased by 

 the addition of many more, and yet most chlorophyll deficiencies 

 are the result of the direct contact of definite genes, probably 

 all of which are recessives. Many genes for albinism or for 

 virescent seedlings have been identified in maize and other plants. 



Another well-known example of the importance of the cyto- 

 plasm is the study of the moss, Funaria, by Wettstein. Hybrids 

 between F. mediterranea and F. hygrometrica resemble the 

 mother in most of their characters. Thus when mediterranea is 

 the female parent, the offspring have small sporocarps with tall, 

 acute opercula, leaf midribs that do not extend out to the apex, 

 leaf apices which are filamentous, and paraphyses which are 

 spiral. In all those characters, these hybrids resemble mediter- 

 ranea. On the other hand, when hygrometrica is the female par- 

 ent, the sporocarps are larger and have broad, flat opercula, the 

 leaf midribs extend out to the apex, the leaf apices are not 

 filamentous, and the paraphyses are not spiral. In these charac- 

 ters, the Fi hybrids resemble hygrometrica. Wettstein back- 

 crossed the hybrids a number of times so as to be certain to 

 have the nucleus of one species in the cytoplasm of the other. 

 No modification of the cytoplasm under the influence of the 

 nucleus of the other species appeared to result even after a num- 

 ber of backcross generations. The conclusion from this work 

 seems without doubt to indicate that the cytoplasm possesses 

 some hereditary potentiality. 



QUESTIONS AND PROBLEMS 



1. Winge has shown that in Taraxacum a great many individuals 

 have arisen that were morphologically distinct. Each individual has 

 multiplied vegetatively and has formed a large clone. These clones are 

 easily recognizable. Should each be considered a separate species? Ex- 

 plain. If not, how should they be considered? Explain. 

 » 2. How can j^ou explain the great difficulty that arises in identifying 

 the species of some genera (such as Crataegus) ? Does that indicate 

 anything concerning the age of the genus? 



3. A population of snails showing some variation is bisected by a 

 small stream which has changed its course. Discuss all the factors that 

 might arise after this geographical isolation had occurred which could 

 result in the origin of new species of snails within the area. Could more 

 than one new species arise on one side of the stream? 



4. What is meant by the age-and-area concept? 



