TRANSACTIONS OF SECTION B, 681 
use of for actual work of assimilation as against 0°5 per cent. in brilliant sun- 
shine.! 
From what I have said previously about the effect of increased tension of 
carbon dioxide on the rate of assimilation, it must follow that the ‘efficiency’ of a 
leaf as regards the permanent storage of energy must, ceteris paribus, be increased 
when small additions of that gas are made to the surrounding air. 
In one such instance, in which the air had been enriched with carbon dioxide 
to the extent of about five and a half times the normal amount, it was estimated 
that the ‘efficiency’ of the leaf for bright sunshine was raised from 0°5 to 2-0 per 
cent. 
Up to the present we have been regarding the efficiency of the assimilatory 
mechanism of a plant in reference to the total energy of all grades which falls upon 
the leaf. It is of course well known that the power of decomposing carbon dioxide 
is limited to rays of a certain refrangibility, and the researches of Timiriazeff, 
Engelmann, and others leave little room to doubt that the rays of the spectrum 
which are instrumental in producing the reaction in the chloroplastids have a dis- 
tinct relation to the absorption bands of the leaf-chlorophyll. By far the greater 
amount of the assimilatory work, probably more than 90 per cent. of it, is effected 
by the rays which correspond to the principal absorption band in the red, lying 
between wave lengths 6,500 and 6,975.2 If, therefore, we express the distri- 
bution of energy in a normal solar spectrum in the form of a curve, we have 
the means of approximately determining the ma.zimum theoretical efficiency of a 
green leaf, that is to say, the maximum amount of assimilatory work which could 
be produced, supposing the conditions so favourable as to admit of the whole of 
the energy corresponding to this absorption band being stored up within the leaf. 
It is not without interest to get an approximate idea of this theoretical 
maximum. 
For this purpose I have here reproduced a curve given by Professor S, P. Langley, 
representing the distribution of energy at the sea-level in the normal spectrum 
of a vertical sun shining in a clear sky. The total amount of incident energy 
represented by the whole area of the curve is 1:7 calories per square centimetre per 
minute, or 1,020,000 calories per square metre per hour. 
I have drawn a thick black vertical band in the red end of the spectrum corre- 
sponding in position and breadth with the principal absorption band of chlorophyll 
as seen in a green leaf. By integration it may be shown that the area of this part 
of the curve is about 6-5 per cent. of that of the whole curve, so that this value 
represents something like the theoretical maximum efficiency of a leaf in bright 
vertical sunshine, supposing the conditions could be made so favourable as to 
' The principal factor which determines the amount of transpiration in a plant 
must be the amount of radiation falling on it. It is essential that the water-bearing 
mechanism should be able to keep up a good supply of water to the leaf lamina in 
order to prevent the temperature rising to a dangerously high point. This ‘ safety 
valve’ function of the transpiration current is not always sufficiently borne in mind, 
and we are too apt to think that the plant requires these enormous amounts of water 
in order to supply itself with the requisite mineral salts. The absolute necessity for 
the supply as a dissipator of energy will become evident by taking one or two facts 
into consideration. A square metre of the lamina of the leaf of a sunflower weighs 
about 250 grams, and its specific heat is about 0-9. We have seen that the hourly 
transpiration in bright sunshine may be as much as 275 c.c. per square metre, re- 
quiring the expenditure of 162,800 calories, and it therefore follows that, if the loss 
of water were stopped, thetemperature of the leaf would rise at the rate of more than 
12° C. per minute. In making our experiments in glazed cases it has sometimes been 
very interesting to watch the result of any accidental stoppage of the water-current 
in the leaf-stalk, and the almost instantaneous effect this has in destroying the leaf 
when the insolation is of high intensity. 
2 These limits are those of the band as measured by passing sunlight through the 
leaf itself. In an alcoholic solution of chlorophyll the band lies between A 6,400 and 
A 6,850. I must here express my thanks to Mr. Charles A. Schunck for having kindly 
undertaken to make these measurements for me. 
