550 Mr. Horace T. Brown [March 22, 



There are two methods by which we can determine the actual 

 amount of atmospheric carbonic acid used up by an assimilating leaf 

 — one a direct, the other an indirect method. 



Part of the apparatus used in the direct method, is shown on the 

 table. 



The leaf, which may he still attached to the plant, is enclosed in 

 a glazed case through which a measured current of air is drawn, 

 of which the carbonic acid content is accurately known. "When the 

 air emerges from the case it passes through an absorption apparatus 

 which retains the whole of the C0 2 left in the air after passing over 

 the leaf. This absorbed carbonic acid is determined at the close of 

 the experiment, and we then have all the data for estimating the 

 carbonic acid abstracted from the air by the leaf. The area of the 

 leaf being known, the C0 2 absorbed can be referred to a unit area of 

 leaf and a unit time. 



By the indirect method, which is due to Sachs, the actual increase 

 in dry weight of a given area of an assimilating leaf is determined, 

 and since this increase in weight is due to substances having a 

 definite percentage of carbon, a simple calculation enables us to 

 determine the equivalent amount of carbonic acid abstracted from 

 the air. 



By such methods as this it can be shown that an actively assimi- 

 lating leaf such as that of the Catalpa tree, in full daylight, and 

 under favourable conditions of temperature, can take in carbonic 

 acid from the air at the rate of about one-tenth cubic centimetre per 

 hour for each square centimetre of leaf. 



Since there are only about three volumes of carbonic acid in 

 10,000 volumes of ordinary air, this must mean that in a single hour 

 the under surface of the leaf will take in as much carbonic acid as is 

 contained in a column of air about eight feet long, and having the 

 same area of cross section as the leaf. 



But this remarkable power of an assimilating leaf will be better 

 appreciated if we compare it with a liquid surface of a strong solution 

 of caustic alkali, which is known to have such a great avidity for 

 carbonic acid. 



We can investigate the absorptive power of such solutions for the 

 carbonic acid of the air under fixed and controllable conditions by 

 using a form of apparatus which I have on the table, and which can 

 be examined at the close of the lecture. It is so arranged that an 

 air current of known velocity can be drawn over the surface of the 

 absorbing solution which has a known area. 



When a very low velocity of the air current has been reached, 

 the amount of absorption becomes constant at ordinary temperatures 

 at about *17 c.c. of carbonic acid, per square cm. of surface per 

 hour. 



So we see that a leaf, assimilating under natural conditions, is 

 taking in carbonic acid from the air more than half as fast as a 

 surface of the same area would do if it were wetted with a constantly 



