56 AERATION AND AIR-CONTENT. 



to 25.7 per cent for carbon dioxid and 1 to 9.5 per cent for oxy- 

 gen. The chief factors determining the amounts are the rate of 

 respiration, the permeability of the peel or skin, and the difference 

 in the pressure of the two gases within and without the tissue. 



Summary. The composition of the internal air of plants depends 

 primarily upon the presence or absence of photosynthesis. Stems, 

 leaves, and other chlorophyllous parts contain air with much oxygen 

 and little carbon dioxid, while roots, tubers, ripe fruits, etc., show 

 much carbon dioxid and relatively small amounts of oxygen. Aim 

 found 17 per cent of oxygen in the internal gas-bubbles of Ulva and 

 21 per cent in the external before sunrise, and these increased during 

 the day to 36 and 55 per cent respectively. No carbon dioxid was 

 present. According to Wille, the bladders of Fucus contained 35 

 to 37 per cent of oxygen in the water, 20 per cent when exposed to 

 the air 10 hours, and but 2.7 per cent after 12 hours in darkness. 

 Saussure obtained 9 per cent of oxygen and 5 per cent of C0 2 from 

 an apple twig, Boussingault, 6.64 per cent of oxygen and 5.34 per 

 cent of carbon dioxid from oleander, and Barthelemy, 10 to 22 per 

 cent of oxygen and to 3 per cent of C0 2 from Nelumbo. Dutrochet 

 found 18 per cent of oxygen in the leaves of Nuphar, Lechartier, 12 

 per cent in the petiole, and Martins and Moitessier, 14 to 15.8 per 

 cent in other water-plants. The air of the interior of the stem of 

 Elodea was shown by Devaux to contain 8.86 per cent of oxygen and 

 2.14 per cent of C0 2 ; the air at the surface of leaves and stems, 23 

 and 0.69 per cent respectively; that of the water, 31 and 21 per cent; 

 and the surrounding air, 18.66 and 0.04 per cent. The air in the cen- 

 ter of a squash was composed of 18.29 per cent oxygen, 2.52 per cent 

 carbon dioxid, and 79.19 per cent nitrogen (cf. Pfeffer, 1900, 1 : 205). 



According to Aubert, the air of succulent plants yielded 18.35 to 

 26.45 per cent of oxygen and 0.47 to 1.50 per cent of carbon dioxid 

 in the sun. Crassula arborescens contained 7 times as much CO 2 in 

 the sun as under a cloudy sky. Dutrochet found but 8 per cent of 

 oxygen in the root of Nuphar, while Magness has recently shown 

 that apples, potatoes, and carrots stored at 11C. and above, regu- 

 larly contain much more CO 2 than oxygen. At 20 to 22 apples 

 averaged 17.2 per cent of carbon dioxid and 5.5 per cent of oxygen; 

 potatoes, 34.4 and 5.7 per cent; and carrots, 28.6 and 5.2 per cent. 



ANAEROBIC RESPIRATION. 



The significance of reduced air-content and of the respiratory prod- 

 ucts arising from it in saturated soils is so great as to warrant a 

 comprehensive account of anaerobic respiration. This lacks com- 

 pleteness chiefly with reference to the theories concerning the 

 nature of intramolecular respiration and fermentation, and to the 

 nature and role of the enzymes concerned. In organizing the large 



