478 
NAD ORE: 
[SEPTEMBER 14, 1899 
different times of the day the varying dry weight of equal areas 
of large leaves, Sachs obtained an approximate measure of the 
rate of the assimilatory process which he could express in terms 
of actual number of grams of substance assimilated by a unit 
area of leaf in unit of time. In thismannerhe was able to show, 
for instance, that a sunflower leaf, whilst still attached to the 
plant, increases in weight when exposed to bright sunshine at 
the hourly rate of about one gram persquare metre of leaf area. 
In the case of similar leaves detached from the plant, and of 
course under conditions in which the products of assimilation 
were entirely accumulated in the leaf, he found an increase in 
weight of rather more than 14 grams per square metre per hour. 
I was able to confirm this work of Sachs in the course of an 
investigation on the Chemistry of Leaves which I made with 
Dr. G. H. Morris in 1892-93, and there can be no doubt that 
the variations in the weight of leaves can be used asa fair index 
of the activity of a leaf in assimilating, but it is not a method 
which admits of much refinement of accuracy, owing, amongst 
other things, to the want of perfect symmetry in the leaves as 
regards thickness and density of the lamine and to the possible 
migration of the assimilated material into the larger ribs, which 
of course cannot be included in the weighings. 
It is evident that a far better plan of measuring the rate of 
assimilation under varying conditions would be the estimation of 
the actual amount of carbon dioxide entering a given area of the 
leaf in a certain time, and it was to the perfection of a method 
of this kind that Mr. Escombe and I first turned our attention. 
In all previous attempts to measure the rate of ingress of 
carbon dioxide, suchas those of Corenwinder, and more recently 
still of Mr. F. F. Blackman, it has been necessary to use air 
containing comparatively large quantities of carbon dioxide, 
amounting to 4 per cent. and upwards. Interesting and useful 
as such experiments undoubtedly are from the point of view 
from which they were undertaken, we must not lose sight of the 
fact that such conditions are highly artificial, and very far re- 
moved from those under which a plant finds itself in the natural 
state. where its leaves are bathed with air containing, not 4 or 5 
per cent., but only ‘03 per cent. of carbon dioxide. I shall have 
occasion later on to show how remarkably the rate of intake of 
carbon dioxide into a plant is influenced by extremely small 
variations in the tension of that gas, and that on this account no 
deduction can be drawn as to the rate of assimilation under 
natural conditions from any experiments in which the air con- 
tains even so small an amount of carbon dioxide as I per cent. 
Before proceeding further in this direction, however, it will be 
well to consider the amount of carbon dioxide which must enter 
a leaf in a given time in order to produce an influence on its 
weight comparable with that indicated by the Sachs method of 
weighing definite areas. For this purpose I will consider a leaf 
with which we have made many experiments—that of Cata/pa 
bignonzoides. \t is a very symmetrical leaf anda good assimilator, 
and since the intake of carbon dioxide takes place only on the 
under side, the question to which I wish to draw your attention 
zan be stated in a simple manner. When such a leaf is subjected 
o a modified form of the half-leaf weighing method of Sachs, 
ito the details of which I cannot here enter, it may, under 
favourable conditions, show an increase in dry weight equal 
to about one gram per square metre per hour. Since this 
increase in weight is due almost entirely to the formation of 
carbohydrates, we can calculate with a close approximation to 
accuracy the corresponding amount of carbon dioxide. This 
will of course depend, within certain narrow limits, on the 
nature of the carbohydrate formed. The formation of a gram 
of starch requires 1°628 grams of carbon dioxide, whilst an 
equal amount of a C,H,,0,% or a CjgH..O,, sugar require 1°466 
and 1°543 grams respectively. From the knowledge we possess 
of the nature of the carbohydrates of the leaf, we are quite sure 
that the mean of these values, that is 1°545 grams, must be very 
near the truth. This amount corresponds to 784 c.c. of carbon 
dioxide at normal temperature and pressure, which must repre- 
sent the volume abstracted by the square metre of leaf surface 
in one hour from air containing only three parts of carbon 
dioxide in 10,000, supposing the method of leaf weighing to 
give correct results. We shall see later on that this intake can 
be verified by direct estimations ; it is equivalent to the total 
amount of carbon dioxide in a column of air of a cross section 
equal to that of the leaf, and of a height of 26 decimetres. 
The extraordinary power which an assimilating leaf possesses 
of abstracting carbon dioxide from the air is best shown by 
comparing it with an equal area of a freely exposed solution of 
NO. 1559, VOL. 60] 
| 
caustic alkali. We have made a very large number of experi- 
ments on the rate at which atmospheric carbon dioxide can be 
taken up by a solution of caustic soda under varying conditions, 
and have been surprised to find how constant the absorption is. 
In a moderately still air a square metre of surface of such a 
freely exposed solution will absorb about 1200 c.c. of carbon 
dioxide per hour, and this can only be increased to about 
1500 c.c. even if the dish is exposed to the full influence of a 
strong wind out in the open. When the surface of the liquid 
is constantly renewed during the experiment by means of a 
mechanical stirrer, the rate of absorption is not sensibly affected, 
providing the agitation does not appreciably increase the surface 
area, and considerable variations in the strength of the alkaline 
solution are also without any effect. On the other hand, slight 
variations in the tension of the carbon dioxide of the air have a 
marked influence on the rate of absorption, and in order to study 
this point we have constructed an apparatus which allows us to 
pass over an absorptive surface of liquid a current of air in 
a stratum of known thickness, and with a known average 
velocity. 
By introducing definite amounts of carbon dioxide into this 
stream of air we have been able to determine the influence of its 
tension on the rate of absorption, At present we have only 
employed air containing amounts varying from o’8 to 13 parts 
per 10,000, that is to say, from about one-quarter to a little 
more than four times the amount contained in normal air. 
Within these limits, and probably beyond them, the rate of 
absorption by the alkaline surface is strictly proportional to the 
tension of the carbon dioxide in the air current. I shall have 
occasion to show later on that the same rule holds good with 
regard to an assimilating leaf, and that in this case also, within 
certain limits, the intake of the gas is proportional to its 
tension. 
The fact which I wish more particularly to bring out in these 
comparisons is that a leaf surface which is assimilating at the 
rate of one gram of carbohydrate per square metre per hour is 
absorbing atmospheric carbon dioxide more than half as fast as 
the same surface would do if wetted with a constantly renewed 
film of a strong solution of caustic alkale. 
From what I have just said about the influence of tension on 
the absorption of carbon dioxide by an assimilating leaf, it is 
clear that any attempts to determine by direct means the natural 
intake of that gas during assimilation must be made with 
ordinary air, and that such experiments can only be carried out 
on a comparatively large scale. We had in the first instance to 
devise an apparatus which would rapidly and completely absorb 
the whole of the carbon dioxide from a stream of air passing 
through it at the rate of from 100 to 200 litres per hour, and at 
the same time admit of an extremely accurate determination of 
the absorbed carbon dioxide. 
The absorbing apparatus which we finally adopted is a modi- 
fication of one used by Reiset in his estimations of the carbon 
dioxide of the atmosphere. It consists essentially of a glass 
tube 50 cm. long, fixed vertically in a wide-mouthed glass 
vessel furnished with a second aperture and tubulure. ‘The 
height of the vertical tube is invariable, but its width is regulated 
according to the amount of air required to be drawn through the 
apparatus in a given time. The bottom of this tube is closed 
with a platinum or silver plate pierced with a large number of 
very small holes, and two other similar perforated plates are 
inserted in the tube at certain intervals. The upper part of the 
tube is put in connection with an aspirating water-pump, and 
the absorbing liquid is placed in the lower glass vessel, whose 
second tubulure is connected with the supply of air in which the 
carbon dioxide has to be determined. When the aspirator is 
started the liquid is first drawn up into the vertical tube, and 
the air then follows through the perforated plates which act as 
*scrubbers."” Reiset, in his work, used baryta water as the 
absorbent, an aliquot part of which was titrated before and 
after the experiment, the changes in the volume of the liquid 
being corrected for by certain devices which I need not 
describe. 
The efficiency of the apparatus as a complete absorber of 
atmospheric carbon dioxide leaves nothing to be desired, but in 
dealing with large quantities of baryta solution, amounting to 
400 c.c. or more, the errors due to inaccurate titrations, or to 
over or under estimation of the volume changes, are all thrown 
on the final result, of which they may form a considerable part. 
We have consequently altogether discarded the use of baryta as 
an absorbent in favour of caustic soda. The carbonate is esti- 
