1919] CURRENT LITERATURE 269 



occasion to collect and determine plants or to study abnormal plant growths. 

 It is the only reasonably complete publication of the kind in America. There 

 are a total of 1439 species, most of which can very readily be recognized by 

 means of the key, the 250 text illustrations, and the 16 full page plates. The 

 pubhcation is indexed both with reference to the host and the parasite. — Mel. 

 T. Cook. 



North American Flora. — The sixth part of volume 22 contains the con- 

 clusion of Rosaceae by Rydberg, including genera 54 to 57, much the largest 

 genus being Rosa, with 129 species, 23 of which are described as new. The 

 part closes with 26 pages of additions and corrections to the volume. 



The first part of volume 32 contains the beginning of Rubiales by 

 Standley, 8 genera of the Rubiaceae being presented. Much the largest 

 genus is Rondektia, with 109 species, 8 of which are described as new. Among 

 the remaining genera 8 new species are described. — J. ^M. C. 



NOTES FOR STUDENTS 



Secondary dormancy in seeds. — Kidd and West^ have continued the 

 study of the controlling action of carbon dioxide on the germination of seeds 

 of Brassica alba. In two previous papers by the senior author it has been 

 shown that low concentrations of carbon dioxide inhibit the germination of 

 seeds, and that temperature and oxygen pressure determine the concentration 

 necessary to inhibit germination. In normal oxygen pressure 2-4 per cent 

 carbon dioxide will inhibit germmation at 3° C, while at 20° C. it requires 

 20-25 per cent. At 17° C. it requires 9-12 per cent carbon dioxide to inhibit 

 with 5 per cent oxygen pressure and 20-25 per cent carbon dioxide with 20 per 

 cent oxygen pressure. All seeds studied, except Brassica alba, germinate 

 normally as soon as the carbon dioxide is removed, while B. alba remains 

 dormant after the carbon dioxide is removed. The authors term this "second- 

 ary' dormancy," in agreement with the usage of this term by Crocker. 



In the production of secondary dormancy the authors note the following 

 general facts: (i) secondary dormancy is not produced if oxygen is absent 

 during the primary period of inhibition or if carbon dioxide has been used in 

 too high concentration; (2) conditions during the primary period of inhibition 

 which prevent subsequent occurrence of dormancy are the ones that exercise 

 injury on the radicle; (3) 100 per cent dormancy is obtained only within 

 narrow limits of carbon dioxide and oxygen pressure. Secondary dormancy is 

 not produced by a change in the permeability of the coats to gases or water, 

 or to an increase in their breaking strength, but by a change in the embryo 



3 Kidd, F., and West, C, The controlling influence of carbon dioxide. The 

 production of secondary dormancy in seeds of Brassica alba following treatment with 

 carbon dioxide and the relation of this phenomenon to the question of stimuli in growth 

 phenomena. Ann. Botany 31:457-487. 1917. 



