12 
INFLUENCE OF SOLAR LIGHT ON VEGETATION. 
instead of empiricism, and prefers scientific accuracy to blind practice. He must 
therefore consider whether he require, for the particular object in view, more 
light, or more heat ; and, in accordance, use a glass of due colour and quality. In 
the cultivation of Orchidace^, for instance, where heat, not light, is usually most 
required, a greyish, or a purplish thick glass, might obviate the necessity of 
shading, or a sheltered situation, and give the plants the sober-toned light which 
they enjoy in their native shades, where they dwell " hidden from day's garish eye." 
The first collections sent to Paris perished through exposure to unmitigated light. 
Most of the plants cultivated in our stoves seem to be sufiiciently furnished 
with light, even where the roof is only imperfectly glazed. In their natural 
situations they generally grow thickly together, their summits only receiving much 
light ; and the vapours that are constantly difi*used (perceptibly or not) through 
a tropical atmosphere, with the immense dilatation of the air by heat, greatly 
modify the light of even a tropical sun ; without considering the rains that accom- 
pany and modify it when vertical. Thus, the banana, the fan-palm, and nume- 
rous other plants, really thrive without injury from the effects of glass. 
There is, however, another class, which, from growing on dry rocks, and 
beneath a hot, clear, and raw sky, require all possible heat and light, as the 
Cactus tribe. But, in the greenhouse tribes of plants, the Azalea, Erica, Camellia, 
Olive, &c., we have to deal with plants natives of climates remarkable for the 
intense purity of their air and consequent brightness of sky and light, though 
without very great heat ; as the cloudless climes of Attica and Provence, and the 
fresh and crisp air of the middle regions of tropical mountain ranges. Such plants 
require more light than glass will transmit, and yet more heat than our open sky 
affords ; and they are consequently the most difficult of culture, either losing their 
leaves or casting their flower-buds. They require a glass that transmits all 
possible light, even to the partial exclusion of heat, for heat can be artificially pro- 
duced : — not so light. 
A classification founded on the natural circumstances of every plant, seems the 
safest method of considering their wants, whether as regards light, heat, air, mois- 
ture, or soil ; of these light is the most difficult to provide for them, in the pro- 
portion in which many require it ; and it; is a nice point to decide how much is 
best for them, in the different stages of their hybernation, active growth, periodical 
sleep, flowering and fruiting states. Nothing but an unwearied course of experi- 
ments can teach this hitherto little known branch of horticulture. The colourino' 
o 
of the fruit, the expansion and closing of the blossoms, the different surfaces of the 
leaf, the motions of the leaf, the twining of tendrils, and the shedding of petals, 
are some of the objects of such experiments. I will conclude with one remark on 
air and moisture, having made so many upon light. 
Unless the water with which plants are supplied is abundantly mixed (in fact 
diluted) with air, it is a fluid too thick to nourish them duly. Hence the great 
superiority of showers over irrigation ; for every drop of rain descends through the 
atmosphere, absorbing air, as a sponge descends through water, filling as it sinks. 
