PHYSIOLOGY. 303 
the mixture, and the green colour is restored to it. 
Almost all known observations on this point prove, 
that leaves, which have parted with their oxygen by 
means of light, are green, but get a pale or whitish 
colour where the oxygen is accumulated. Chemists 
now mostly assign as a cause of the green colour of 
plants, the particular proportion in which the hydro- 
gen and carbon are mixed. 
6 282. 
The dark colour of the bark im woody plants is, 
according to Berthollet’s observations, produced by 
the oxygen of the atmosphere. Mr Humboldt re- 
peated his experiments, and found that wood, when 
inclosed in oxygen gas, became black in two or 
three days, and the gas was mixed with carbon. It 
appears from this, that the oxygen of the atmosphere 
combines with the hydrogen of the vegetable fibre, 
and sets the carbon free, which shows its particular 
black colour. 
§ 283. 
The duration of the leaves of plants varies very 
much. Most of them in warm climates remain from 
‘ three to six years on the stem. A few in colder 
climates, and only those which have a tenacious 
sap, as Llex aquifolium and Viscum album, or such, 
which have sap of a resinous nature, as all the 
pine-tribe trees, retain their leaves during winter. 
All other plants of the colder climates drop their 
leaves inautumn. ‘This happens in many different 
ways. Some leaves shrink gradually together, fall off, 
3 or 
