PERIODS OF REST. 369 



of pressure on the various functions, by which it appears that in 

 ordinary air, under a pressure of six atmospheres, Mimosa per- 

 islied quickly. In an atmosphere under high compression seeds 

 germinated, if at all, very slowlj'. 



&74. Influence of temperature upon respiration. Respiration 

 can go on at low temperatures, even near the freezing-point of 

 water. The rate of respiration increases with rise of tempera- 

 ture, as will be seen from the following figures for germinating 

 beans : — ^ 



Tpmiicratiirfi Amount of carbonic add 



±emi)erature. given off each hour. 



2°C 10.56 mgi-. 



6° 21.22 " 



18° 32.34 " 



20° 39.60 " 



30° 47.52 " 



975. Influence of liglit upon respiration. It is not jet known 

 positivel}' whether light has any effect upon respiration. In 

 some experiments there has been a sHght increase,^ in others a 

 diminution,^ in the rate, with increased illumination ; but it is 

 not certain whether all other factors wei-e excluded. 



If the produced carbonic acid does not escape readily from the 

 tissues, respiration goes on more slowly.* 



976. Periods of rest. Although all plants require oxj'gen for 

 the performance of their normal functions, it by no means follows 

 that when a plant is supplied with oxj'gen the normal activities 

 will be necessarily exhibited. In the case of certain bulbs, seeds, 

 etc., even with the most favorable surroundings, there maj' be 

 no signs of respiratory or other activity until after the lapse of 



1 Rischawi; Versuchs-Stationen, xix., 1876, p. 338. 



2 Wolkoff and Mayer; Landwirtlischaftliche Jalirbiicher, 1874, Heft iv. ; 

 Cahours : Cornptes Reiidus, Iviii., 1864, p. 1206. 



^ Dumas: Annales de Chiiiiie et de Physique, ser. 5, tome iii., 1874, 

 p. 105 ; Borodin : Just's Botan. Jahresbericht, iv., 1878, p. 920. 



* For the bearings of tliis upon alooliolic fermentation, which, according to 

 Melsens, is not arrested until a pressure of 25 atmospheres of carbonic acid 

 is reached, see Pa.steur : Annales de Chimie et de Pliysique, ser 3, tome Iii., 

 1858, p. 415; and Nageli: Die niederen Pilze, 1877, p. 31. 



Alcoholic Fermentation. This process is so intimately connected witli 

 that of respiration that it requires a brief desci'iption at this point. Reduced 

 to its simplest terms, it consists of the clianges which are produced in a solu- 

 tion of sugar by the growth of a microscopic organism. This is some one of 

 the Saccharomycetes (a group of low fungi which are propagated by a process 

 of budding). By the growth of this fungus the solution of sugar is broken up 

 into various products, the most noteworthy being alcohol and carbonic acid. 



24 



