PHYSIOLOGY. 293 
Mr Humboldt found that the leaves of the Lepidium 
sativum, in the faint glimmer of a lamp, which was 
kept up for some days, retained their green colour. 
I saw myself this singular and remarkable pheno- 
menon. 
Hydrogen gas likewise promotes the decomposi- 
_ tion of water in vegetables. Sennebier and Ingen- 
houss observed that plants, inclosed in hydrogen gas, 
transpired day and night oxygen gas. Mr Hum- 
boldt on the 14th February 1792 took a germinating 
bulb of the Crocus vernus down to one of the cele- 
brated mines of Freyberg, and planted it in the 
ground. In its galleries the air was so much con- 
taminated with hydrogen gas, that his candle went 
out, and his lungs became sensibly affected. ‘The 
germ of the bulb soon evolved its leaves and flowers. 
Till the 17th day the leaves were green, the flowers 
yellow, and the anthers even full of pollen; but on 
this day the whole plant began to putrify. Several 
plants shewed the same result. The hydrogen gas 
cannot however be considered as a stimulus of vege- 
tables, as in its pure state it kills plants, and only 
when mixed with oxygen shews the above pheno- 
mena. Plants therefore remain alive in it as long 
only, as they can still exhale oxygen; when this 
stops the plant is gone. . 
Oxygen gas is therefore, as experience shews, as 
exclusively necessary to the subsistence of plants as 
of animals. Its stimulus of the vegetable fibre is 
that which preserves the health of plants; and 
therefore plants grow rapidly when they can ims 
bibe oxygen gas from the ground, Seeds like- 
T3 wise 
