DEPARTMENT OF BOTANICAL KESEAKCH. 83 



of the root. In roots that are relatively short, 3 cm. more or less in 

 length, growth goes on for a short time in an apparently normal man- 

 ner in an atmosphere containing 2.67 per cent oxygen, but in roots 

 which are relatively long — for example, 10 cm. — growth soon ceases 

 in an atmosphere containing this amount of oxygen. On the other 

 hand, growth proceeds in the longer roots for 5 days, more or less, in 

 an atmosphere containing 4.56 per cent oxygen. It seems probable, 

 therefore, that after germination has started in Prosopis, root-growth 

 may go on for a considerable period under practically anaerobic condi- 

 tions. How long this takes place was not accurately determined, but 

 may possibly be related to the duration of the cotyledonary food-supply. 



In a soil atmosphere containing carbon dioxide and oxj^gen the 

 results have been somewhat at variance. It appears, however, that 

 relatively long roots, for example, 20 cm., can grow for at least 24 

 hours in an atmosphere containing 25 per cent oxygen and 75 per cent 

 carbon dioxide, although growth ceases after this period. Roots 

 approximately 5 cm. in length continue growth in 90 per cent carbon 

 dioxide during administration, although this was not always the case. 

 In all cases where growth stopped, or where the rate was decreased, it 

 was renewed sooner or later after atmospheric air had replaced the gas 

 used. Thus the specific effects of carbon dioxide, even with a large 

 amount of oxygen, are well marked. 



In Opuntia versicolor growth in all cases stopped promptly in an 

 atmosphere of 2.67 per cent oxygen. Roots 3 to 7 mm. in length 

 stopped growth in an atmosphere containing 4.56 per cent oxygen, 

 although it was seen that roots 11 cm. in length maintained a good 

 growth-rate for 48 hours in the same atmosphere when growth ceased. 

 It appears, therefore, that at least the shorter roots of Opuntia cut- 

 tings have a greater oxygen requirement than the longer roots of Pro- 

 sopis seedlings, but that a differential result also occurs. It remains 

 to be seen whether the latter is associated with the well-known differ- 

 ential development of the roots of the species into shallow absorbing 

 and more deeply placed anchoring roots, as appears to be indicated, 

 although this is suggested. 



Rate and Course of Growth of Succulents, by D. T. MacDougal. 



The construction of a dozen auxographs of improved pattern (see 

 Ann. Report, Dept. Bot. Research for 1916, p. 57, Carnegie Inst. 

 Wash. Year Book) has made it possible to secure an amount of infor- 

 mation upon which some important conceptions of growth may be 

 based. The annotated records of growth now available at the Desert 

 Laboratory probably comprise more than half the existing information 

 on the subject. The changes in length of an individual joint of an 

 Opuntia for 16 months are included in the data available. jMeasure- 

 ments of growth of the massive succulents was continued with Echino- 



