482 
the absorption rate of an equal area of a freely exposed surface 
of a solution of caustic alkali, we arrive at the conclusion that, 
under the particular conditions of our experiment, the diffusion 
rate through an aperture of I mm. is forty ¢imes greater than the 
rate of absorption of a free alkaline surface of equal area. 
This corresponds to an actual average rate of passage of the 
molecules of the atmospheric carbon dioxide of about 266 centi- 
metres per minute. 
Now, we have already seen, in the case of a Catalpa leaf, 
that if the gaseous exchange during assimilation goes on only 
through the stomatic openings, we require a minimum velocity 
of something like 380 centimetres per minute, a velocity which 
we are sensibly approaching in our experiments with apertures 
of about I mm. in diameter. But the effective area of a stomatic 
opening of the Catalpa leaf is equal to that of a circle witha 
diameter of less than 1/100 mm., and since our experiments in- 
dicate a very rapid increase in the velocity of diffusion as the 
aperture is diminished, it is clear that no difficulty, as regards 
the physics of the question, can be raised against the idea that 
atmospheric carbon dioxide reaches the active centres of 
assimilation by a process of free diffusion through the leaf 
stomata. 
One of the most interesting problems connected with plant 
assimilation relates to the efficiency of a green leaf as an 
absorber and transformer of the radiant energy incident 
upon it. 
It is already well known that the actual amount of energy 
stored up in the products of assimilation bears a very small 
proportion to the total amount reaching the leaf: in other 
words, the leaf, regarded from a thermo-dynamic point of view, 
is a machine with a very low ‘‘ economic coefficient.” We 
» 
Fic. 3. 
require, however, to know much more than this, and to as- | 
certain, amongst other things, how the efficiency of the machine | 
varies under different conditions of insolation, and in atmospheres 
containing varying amounts of carbon dioxide. 
The measure of the two principal forms of work done within | 
the leaf, the vaporisation of the transpiration water on the one | 
hand, and the reduction of carbon dioxide and water to the 
form of carbohydrates on the other, can be ascertained by 
modifying our experiments in such a manner as to allow the | 
transpiration water to be determined, as well as the intake of 
carbon dioxide. 
For the actual measurement of the total energy incident on 
the leaf under various conditions we are now using one of Prof. 
-Callendar’s recording radiometers of specially delicate con- | 
struction, which will be ultimately calibrated in calories, This 
instrument gives promise of excellent results, but up to the | 
present time the work we have done with it is not sufficiently 
advanced for me to describe. We may, however, obtain a very | 
fair idea of the variation in the efficiency of a leaf from one or | 
two examples in which the amount of incident energy was 
deduced from other considerations. 
In the case of a sunflower leaf exposed to the strong sunlight 
of a brilliant day in August, the average amount of radiant 
energy falling on the leaf during the five hours occupied by the | 
experiment was estimated at 600,000 calories per square metre | 
per hour. The averege hourly transpiration of water during | 
that time was at the rate of 275 cc. per square metre, and the 
assimilated carbohydrate, estimated by the intake of carbon 
dioxide, was at the rate of 08 gram per square metre per hour. 
The vaporisation of 275 c.c. of water must have required the | 
expenditure of 166,800 calories, and the endothermic pro- 
duction of o°8 gram of carbohydrate (taking the heat of | 
NO. 1559, VOL. 60] 
NALTORE 
“1 09 190 WO 120 130 140 150 160 170 180 190 200 210 220 229 240 250 260 270 280 
| weighs about 250 grams, and its specific heat is about o’9. 
[SEPTEMBER 14, 1899 
combustion at 4000 gram calories) corresponds to the absorp- 
tion of 3200 calories. Thus, as the final result under these 
particular conditions of experiment, we find that the leaf has 
absorbed and converted into internal work about 28 per cent. of 
the total radiant energy incident on it, 27°5 per cent. being used 
up in the vaporisation of water, and only one-half per cent, in 
the actual work of assimilation. 
In strong diffuse light, such as that from a northern sky ona 
clear summer’s day, the leaf has a higher ‘‘economic co- 
efficient,” using that term in relation to the permanent storage 
of energy in the assimilatory products. In one instance of this 
kind in which the total energy received by the leaf was 
approximately 60,000 calories per square metre per hour, it was 
found that 96 c.c. of water were evaporated and ."41 gram of 
carbohydrate was formed for the same area and time. This 
indicates an absorption and utilisation by the leaf of something 
like 95 per cent. of the incident energy, of which 2°7 per cent 
has been made use of for actual work of assimilation as against 
0°5 per cent. in brilliant sunshine. ! 
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, caeter?s parzbus, 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 20 per cent. 
Up to the present we have been regarding the efficiency of 
the assimilatory mechanism of a plant in reference to the /ofa/ 
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 refran- 
gibility, and the researches of Timi- 
riazeff, Engelmann and others leave 
little room to doubt that the rays 
of the spectrum which are instru- 
mental in producing the reaction in 
the chloroplastids have a distinct 
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 6500 and 
6975." If, therefore, we express 
the distribution of energy in a normal solar spectrum in the 
form of a curve, we have the means of approximately deter- 
mining the maxzmum 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 
Prof. S. P. Langley representing the distribution of energy at 
the sea-level in the normal spectrum of a vertical sun shining in 
1 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 goud supply of water 
to the leaf lamina in order to prevent the temperature rising to a danger- 
ously high point. This ‘‘ safety valve” function of the transpiration cur- 
rent 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 willjbecome evident by taking one or two facts 
into consideration. A square metre of the lamina of the leaf of a sunflower 
We have seen 
that the hourly transpiration in bright sunshine may be as much as 275 c.c. 
per square metre, requiring the expenditure of 162,800 calories, and it 
therefore follows that, if the loss of water were stopped, the temperature of 
the leaf would rise at the rate of more than 12° C. fer 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 6400 and A 6850. I must here express my thanks to Mr. 
Charles A. Schunck for having kindly undertaken to make these measure- 
ments for me. 
