230 



FUNDAMENTALS Of CYTOLOGY 



This supports the conckision that the characters involved have a differen- 

 tial basis in the cytoplasm. 



A comparable example is furnished by certain moss hybrids, in which 

 gametophytic characters are involved. When Funaria hygrometrica and 

 Physcomitrium pyriforme are crossed reciprocally, the resulting hybrid 

 sporophytes show characteristic structural differences, as do the diploid 

 hybrid gametophytes produced from them by regeneration. Some of 

 these gametophytic characters, notably the length of the leaf midrib, per- 

 sist throughout subsequent backcross generations of monoploid offspring 

 grown from spores, indicating a strong and persistent cytoplasmic effect. 



The shape of the leaf shows the effect some- 

 what less strongly, and the form of the 

 paraphyses shows it still less. The conclusion 

 is that in these mosses, and presumably in 

 other plants showing strongly persistent 

 cj^toplasmic effects, there is a stable element 

 in the cytoplasm that acts differentially upon 

 characters. This element in mosses is called 

 the plasmone. In the case of some characters, 

 such as midrib length in the above-mentioned 

 hybrids, it is the plasmone that is responsible 

 for differences in the character, while other 

 characters may be acted upon differentially 

 by both plasmone and genome or by the 

 genome alone. 



Peculiar interest attaches to a case of 

 cytoplasmic inheritance in Zea because of a 

 visible difference in the cytoplasm correlated 

 with the character involved. In a certain 

 race of maize the pollen degenerates partially 

 or completely, usually after the formation of the generative cell. This 

 character, male sterile, is transmitted through the eggs to the next genera- 

 tion, but not by the few good pollen grains from partially sterile plants. 

 That the cause of the defect is actually in the cytoplasm and not in the 

 genes has been shown by a series of crosses involving each of the 10 chro- 

 mosomes. In cells that are to ^deld normal pollen, certain bodies, pre- 

 sumably the proplastids, are rod-shaped, whereas in cells about to 

 produce degenerating pollen they are spherical (Fig. 169). These bodies, 

 if not the cause of the defect, are at least indicators of a determining 

 influence in the cytoplasm. 



Chlorophyll Characters. — Two successive generations of cells repro- 

 ducing by division resemble each other partly because the organs of a 

 given cell may actually continue as the corresponding organs of its daugh- 



h'lo. 169. — Two sporocytes 

 in a partially male-sterile indi- 

 vidual of maize. Normal cell 

 above, affected one below. 

 See text. {Courtesy of M. M. 

 Rhoades.) 



