8 NATURAL CONDITIONS OP PLANTS. 
Man makes use of these facts in rendering 
many plants available for food, that could not 
otherwise be eaten, as the endive, celery, &c. 
“ In North America, the operation of light in 
colouring the leaves of plants, is sometimes 
exhibited on a great scale, and in a very striking 
manner. Over the vast forests of that country 
clouds sometimes spread, and continue for many 
days, so as almost entirely to intercept the rays 
of the sun. In one instance, just about the 
period of vernation, the sun had not shone for 
twenty days, during which time the leaves of 
the trees had reached nearly their full size, hut 
were of a pale or whitish colour. One fore- 
noon the sun broke through in full brightness, 
and the colour of the leaves changed so fast, 
that, by the middle of the afternoon, the whole 
forest, for many miles in length, exhibited its 
usual summer’s dress.” — Ellis. 
MOISTURE. 
Without moisture, there can be no vegetation. 
Whatever may be the degree of heat, or of cold, 
or deficiency of light, if there be but moisture, 
plants of some kind are to be found. They 
form the oases in the sandy deserts, vegetate in 
the snow of the Arctic regions, and in and on 
