Genetic Factors 



427 



inbred and hybrid maize by Weaver (1946), Heimsch, Rabideau, and 

 Whaley (1950), and Stein (1956); in a leaf mutant in maize by Mericle 

 ( 1950 ) ; and in the leaves of two species of Tropaeolum and their hybrid 

 by Whaley and Whaley ( 1942 ) . In this last piece of work it was found 

 that the pattern was essentially determined by early differential cell 

 division in certain regions, the final form being attained by uniform 

 cell expansion. Much more work of this sort, even at the purely descrip- 

 tive level, needs to be done, for it will doubtless pose more sharply the 

 problems which have to be solved and may suggest new methods of 

 attacking them. 



Fig. 19-6. Acetabularia. A, A. mediterranea. B, A. wettsteinii. C, a stalk piece (shaded) 

 of the former species grafted to a rhizoid of the latter, which contains the nucleus. The 

 regenerating "hat" resembles that of A. wettsteinii and thus seems to be determined 

 by the nucleus. ( After Hiimmerling. ) 



Acetabularia. In a few cases more direct proof of gene control over 

 form characters has been obtained. One of the most notable of these is 

 presented by the coenocytic marine green alga Acetabularia (p. 136). This 

 plant has a branching, rhizoidal base from which rises a stalk several 

 centimeters high, surmounted by a "hat" something like the "umbrella" 

 of a mushroom. At most stages of its life history, Acetabularia has but a 

 single, large nucleus, located in one of the basal rhizoids. Two species 

 of the genus differ in size and especially in the form of the hat. If a 

 long piece of the stalk of the taller species ( A. mediterranea ) is cut out 

 and grafted to a decapitated basal portion of the other (A. wettsteinii), 

 which contains the nucleus, a new hat will be regenerated at the apex 

 of the grafted stalk. At first this hat will resemble that of the species 

 which contributed the stalk but at length it comes to be like that of the 



