318 PHYSIOLOGY OF NUTRITION. 



with ice, and instead of having an ordinary lid the apparatus is 

 closed by a zinc tray filled with ice. The potatoes are thus ex- 

 posed in the thermostat to a constant temperature of C. For 

 further information respecting such a thermostat see 49. If pota- 

 toes which contain little or no sugar are placed in a glass vessel, 

 this in a freezing mixture (snow or ice and common salt), so that 

 the potatoes rapidly freeze, and become ringing hard, there is no 

 change in the quantity of sugar present, as is indicated by grating 

 them down while frozen, and extracting with water. These facts 

 were first demonstrated by Miiller-Thurgau. 1 A few observations 

 on the subject are also to be found in a short treatise published by 

 me. 2 Miiller-Thurgau has found that the relation between the 

 processes of sugar formation and respiration in potatoes is very 

 different at different temperatures, and that it is very important 

 in explaining the phenomena observed to keep this in view. At a 

 high temperature (say 15-20 C.) respiration proceeds compara- 

 tively energetically, so that the sugar is used up as fast as it is 

 formed, and cannot accumulate in the potatoes. At a lower tem- 

 perature {e.g. 0-3 C.) more sugar is formed from the starch 

 present than can be made use of in the now feeble respiration. 

 Consequently at lower temperatures accumulation of sugar takes 

 place. The mere freezing of potatoes is without influence on the 

 quantity of sugar they contain. For observations on respiration 

 see 10'2. 



1 See Miiller-Thurgau, Landwirthschl. Jahrbiicher, Bd. 11, p. 751. 



2 See Detmer, Pflanzenphysiologisclie Untersuchungen ilbcr Fermentbilduny 

 tin d fermentative Processe, 1884, p. 41. 



127. The Ripening of Fruits and Seeds. 



If a thin transverse section from a ripe seed of Brassica Napus 

 is examined, it is seen that the seed-coat is made up of a number 

 of different layers. Passing from the outside inwards we have 

 first a colourless layer consisting of compressed cells, followed by 

 a layer consisting of brown-coloured cells, the lumina of which 

 are fairly distinct. Upon this abuts a layer composed of brown 

 compressed cells, followed by a fourth whose cells are strongly 

 thickened, and exhibit distinct lumina, while the fifth layer, like 

 the first, no longer presents cellular structure. The second and 

 third layers of the seed-coat cause the brown coloration of the 

 seed. In the cells of the folded cotyledons appear large quantities 



