i92o] CURRENT LITERATURE 325 



* 



The other islands about New Zealand also supply similar data. 18 In 

 studying these floras Willis contends that through this hypothesis one is 

 able to prophesy that the plants which reach outlying islands will be on the 

 whole the oldest, and therefore the most widespread upon the mainland, and 

 finds, on examining the facts, that the prophecy is completely fulfilled. The 

 facts presented seem to support the contention and lead the author to restate 

 the hypothesis thus: "The area occupied at any given time, in any given 

 country, by any group of allied species at least ten in number, depends chiefly, 

 so long as the conditions remain reasonably constant, upon the age of the 

 species of that group in that country, but may be enormously modified by 

 the presence of barriers such as seas, rivers, mountains, change of climate 

 from one region to the next or other ecological boundaries, and the like, also 

 by the action of man, and by other causes. In other words, age and area is 

 the chief positive, the action of barriers the chief negative, factor in plant dis- 

 tribution, while in recent times the action of man has become of greater 

 importance than either." — Geo. D. Fuller. 



Gases and germination. — Kidd 19 has studied the effect of various partial 

 pressures of carbon dioxide and oxygen upon the sprouting of potatoes, and 

 concludes that "(i) Oxygen is harmful to the potato tuber in concentration 

 of about 5-10 per cent; oxygen 80 per cent kills in 4-5 weeks; oxygen 

 5-10 per cent is the optimal concentration for sprouting. (2) The harmful 

 action of oxygen is increased in the presence of carbon dioxide. (3) Carbon 

 dioxide inhibits sprouting in a concentration of 20 per cent. This concen- 

 tration is at the same time to some extent harmful. (4) Higher concentra- 

 tions of carbon dioxide cause marked injury and death." Nobokirch has 

 found that actively growing plant organs grow faster, in oxygen pressures 

 considerably below that of the normal atmosphere, but that such reduced 

 pressures finally prove injurious, due to accumulation of metabolic products; 

 while at normal oxygen pressures no such injury occurs. This may throw in 

 question Kidd's interpretation that pressures above 10 per cent are injurious, 

 especially for pressures up to the normal atmosphere. In general, due to 

 their coats and other coverings, seeds are reduced in rate and percentage of 

 germination by any reduction of oxygen pressure below the normal atmos- 

 phere, and often favored by greater oxygen pressures. Some of the work of 

 Appleman has indicated that oxygen supply is a limiting factor to germination 

 * of the potato, quite in contrast with Kidd's results. It is interesting that 



carbon dioxide showed no forcing action due to its anaesthetic properties. It 

 is possible that it did cause increases in respiration, while not increasing or 



18 Willis, J. C, The floras of the outlying islands of New Zealand and their 

 distribution. Ann. Botany 33:267-293. 1919. 



19 Kidd, Franklin, Laboratory experiments on the sprouting of potatoes in 

 various gas mixtures. New Phytol. 18:248-252. 1919. 



