112 
always to be accompanied by the retention of that 
gas ? Could we discover the connection between these 
facts, it might, perhaps, lead us to an explanation of 
the cause of the green colour of plants. 
354. As we have before endeavoured to shew 
(338.), that the decomposition of carbonic acid by 
plants cannot properly be deemed a vegetative func- 
tion, so likewise the changes which plants exhibit, in 
passing from an etiolated to a coloured state, and the 
circumstances under which these changes take place, 
equally prove their independence of a living action. 
When an etiolated plant is gradually exposed to light, 
we first observe, says M. Senebier, the most tender 
parts pass from a white to a yellow colour ; the yel- 
low then becomes deeper ; next, some green spots, 
are seen at the extremity, and on the borders of the 
leaf, and in the angles of its nerves. These spots 
multiply, extend, and meet ; the stalk of the leaf af- 
terwards becomes green, and, lastly, the stem. The 
new leaves are green from the first, and thus upon 
the same stem we may see some leaves very green, 
others much less so, and a stem that is still white. 
In young leaves, the extremity is often greener than 
the other parts ; and in the narcissus, he often saw 
the point of the leaf more green than the rest, and 
the plant, which had just issued from the bulb, less 
green than that which had been some days exposed ; 
whence it seems to result, says M. Senebier, that the 
green colour depends absolutely on a- combination 
effected exteriorly, and which, to a certain extent, is 
independent of the internal vegetation of the bulb *, 
* Mem. Phys. Chim. t. ii. p. 88. 90. 
