JOURNAL 
OF THE 
Royal Horticultural Society. 
Vol. XXIII. 1899. 
Paet II. 
. ON THE IMPORTANCE OF LIGHT AND HEAT TO PLANTS. 
Lecture at Cbiswick Gardens. 
By Rev. Prof. G. Henslow, M.A., V.M.H., &c. ' 
[June 7, 1899.] 
Obvious Effects of Light. — The most universally conspicuous 
effect of light upon ordinary flowering plants, ferns, &c., is the greening 
caused by it ; for if light be excluded, blanching occurs, the plants being 
white or yellowish. All rays of the luminous portion of the solar spectrum 
will cause iDlants to be green, though not quite so strongly as when they 
are exposed to pure light. There is, however, an optimum amount of 
light for every plant. This is easily seen in the leaves of Aacuha japonica, 
which are of a deep green within the bush, but more or less spotted with 
yellow on the circumference. The yellow becomes more uniform on a 
very bright summer's day. Similarly young conifers turn to a yellowish 
green if the light be too great. "When the difference is not visible to the 
naked eye, the spectroscope reveals it.* 
Forms of Leaves. — As a general rule broad leaves are exposed 
horizontally — i.e., more or less at right p.ngles to incident light ; while 
narrow or linear leaves are approximately vertical. The foliage of our 
common timber trees. Oak, Elm, Lime, &c., illustrate the former; while 
herbs that naturally grow thickly together or tufted have erect leaves, as 
Grasses, Sedges, Pinks, Thrift, and certain species of genera, which differ 
from the rest in this particular, as Baiuinculns graimneiis, Stella ria 
Holostea, Latliyrus XissoUa, kc, which grow^ among Grass or other 
foliage, which necessitates an erect habit. 
But the difference may occur on one and the same plant, the early- 
formed leaves being broad and horizontal, but those on the stem linear 
* See Mr. Sorby"s experiments alluded to in Orirjin of Plan: Structiircs, p. llo. 
B 
