chap, in.] SOWING SEEDS. 
47 
air, or they become yellow and withered. 
Light absorbs the oxj^gen from plants, and 
occasions a deposition of the carbon. Thus 
seeds and seedlings do not require much 
light; it is indeed injurious to them* as it 
undoes in some degree what the air has been 
doing for them: but young plants, when they 
have expanded two or three pairs of leaves, 
and wdien the stock of carbon contained in 
their cotyledons, or seed-leaves, is exhausted, 
require light to enable them to elaborate 
their sap, without which the process of ve¬ 
getation could not go on. Abundance of 
light also is favourable to the development 
of flowers, and the ripening of seeds; as it 
aids the concentration of carbon, which they 
require to make them fertile. The curious 
fact that seeds, though abundantly supplied 
with warmth and moisture, will not vegetate 
without the assistance of the air, was lately 
verified in Italy; where the Po, having over¬ 
flowed its banks near Mantua, deposited a 
great quantity of mud on some meadows; 
and from this mud sprang up a plentiful crop 
of black poplars, no doubt from seeds that 
had fallen into the river from a row of trees 
