THE USES OF LEAVES. 
95 
THE USES OF LEAVES. 
Substance of a Lecture at Chiswick Gardens. 
By Rev. Prof. G. Henslow, M.A., V.M.H., &c. 
[July 10, 1900.] 
To execute their functions properly leaves must be green. If they are 
otherwise coloured, as of Coleus and Copper-beech, then the green chloro- 
phyll, though present, is obscured. 
If they are yellow or white in places or variegated, such is usually an 
abnormal condition of things, and due to insufficient light or want of iron 
or other ingredient in the soil. 
In the case of many parasitic plants, as Dodder, Broomrape, and 
Toothw^ort, these are greenless because, livnig on host-plants, they da 
not require any chlorophyll at all for assimilating purposes. 
If one trace the changes undergone by protoplasm from an etiolated 
condition in darkness to a green one in light, it is found that the colour- 
less protoplasm breaks up into pieces which become green by the action 
of light. The green colouring matter can be artificially extracted when 
the colourless protoplasmic granules remain. 
A green leaf is thus prepared to perform two functions, viz. assimila- 
tion and transpiration, under the action of sunlight. To understand 
Avhat takes place, a three-sided prism of glass is required to decompose a ray 
into the coloured spectrum ; and if this is placed before a clear green 
solution of chlorophyll dissolved in spirits of wine, seven dark bands 
appear across it. One very black is in the red, three smaller and lighter 
to the right of the last in the orange and yellow, and three very broad 
and dark in the blue and violet end. 
The meaning of this is that the light corresponding to the position of 
these bands is stopped and absorbed by the chlorophyll solution. 
No bands occur in the green light ; hence the green rays are not 
absorbed, but are either reflected from the surface of the leaf or trans- 
mitted through it. That is why leaves are green. 
To find out which of the rays are specially concerned in these two 
functions various experiments have been made, as by growing plants 
under coloured glasses or coloured solutions. Unfortunately no glass, 
except perhaps a ruby red one, is pure," in that they transmit the rays 
which give the colour, but the others are not entirely stopped. They aie 
only obscured by the predominant colour, and so are invisible to the eye. 
The spectroscope, however, at once detects their presence. Hence all 
such experiments can only give approximate results. 
Now it is found that although assimilation goes on more or less under 
all coloured glasses, they show certain maxinia and minima. The be^t 
for assimilation are yellow and blue, while for transpiration, red, violet, and 
(feebly) green are the colours under which the greatest amount of water 
is given off. Hence it appears that the rays more especially concerned 
with these functions alternate with each other.* 
The reader is referred to my paper in Journal of the Royal HorticuUinal Socicti/. 
vol. xvi., 1893, p. 59. 
