I. RESPIRATION AND OXYGEN. 

 NORMAL RESPIRATION OF ROOTS. 



The necessity of oxygen for the proper functioning of roots has 

 notf only been repeatedly demonstrated by direct experiment, but 

 further evidence of it has also been gained from the behavior of other 

 organs. This would be expected from the fact that the basic re- 

 sponses of protoplasm are more or less identical for all green plants. 

 As a consequence, studies of the oxygen requirements of leaves and 

 shoots have likewise much significance for the behavior of roots. 

 In the case of underground shoots, rootstocks, tubers, bulbs, and 

 corms, the process of respiration is not only identical with that of 

 roots, but the relation to the air-content is also the same. This is 

 true to a large extent of all soil organisms and especially of the host 

 of aerobic fungi. Soil algae, on the contrary, free more oxygen than 

 they use and serve to maintain the air-balance of the soil. 



In organizing the evidence derived from the results of many inves- 

 tigators, it has been desirable to preserve the historical sequence as 

 far as possible, but at the same time to give a coherent discussion 

 of the various phases of the subject. In consequence, our knowledge 

 of the oxygen requirements of roots is divided into three sections, 

 namely, normal respiration, anaerobic respiration, and field studies 

 of aeration. Each of these furnishes its own particular body of 

 evidence and all must be taken into account for a complete view. 

 For convenience, the first two have been divided into three general 

 periods determined by the years 1870 and 1900. One of these 

 marks the first studies of intramolecular respiration by Pfeffer and 

 his students, and the other the beginning of quantitative ecology, 

 as well as a more intensive attack upon the problems of respiration 

 by Palladin, Stoklasa, Nabokich, and their associates. 



Early researches. The importance of oxygen for germination and 

 growth was first demonstrated toward the close of the seventeenth 

 century by several investigators. The first of these were Mayow 

 (1668), who found that oxygen was indispensable to plants, and 

 Huygens and Papin (1674), who showed that plants die under the 

 air-pump in the absence of air. Malpighi (1687) was the first to 

 determine that air was required for germination, while Ray (1690) 

 discovered that lettuce seeds would not germinate in a vacuum, but 

 did so readily upon the return of air. Homberg (1699) found that 

 the seeds of Portulaca oleracea, Lactuca saliva, and Lepidium sativum 

 germinated slowly or not at all in rarefied air. 



Hales (1727) first discovered that CO 2 was secreted by roots. 

 Corti (1774 : 210) placed plants of Chara in a vacuum and left them 



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