Notes. 
164 
must be allowed after each double estimation, during which the 
titrations are performed ; the currents of air in connexion with the 
plants then pass through bye-paths, still at their previous rate. This 
interval, in which a double titration, emptying and refilling of the 
absorption chambers, is accomplished can be reduced to ten minutes. 
Delicacy of estimation sufficient for present work is obtained by 
the use of half-decinormal, N/20, standard solutions. Phenolphtha- 
lein is used as indicator, and specially delicate end-reactions can be 
obtained, since atmospheric C 0 2 is excluded, and moreover the 
burettes containing both the solutions can be drawn upon. 
The burettes, narrow and graduated in 1/10 c.c., are read to 
1/100 c.c., with a simple arrangement for avoiding parallax. All 
other usual precautions are taken, and series of control titrations, 
with an error of observation of not more than o-i per cent., are often 
obtained. This corresponds to 1/200 c.c. C 0 2 . 
In experiments of short duration, 1/50 c.c. C 0 2 is found to be suffi- 
cient for a trustworthy estimation from which definite conclusions may 
be drawn. 
The communication immediately following the present one, illus- 
trates the applicability of this apparatus to the investigation of minute 
quantities of carbon dioxide. 
EXPERIMENTAL RESEARCHES ON VEGETABLE ASSI- 
MILATION AND RESPIRATION. NO. 2. ON THE PATHS 
OF GASEOUS EXCHANGE BETWEEN AERIAL LEAVES 
AND THE ATMOSPHERE \ By F. F. Blackman, B.Sc., Demon- 
strator of Botany in the University of Cambridge. — On the question 
of the path by which carbonic acid passes out of the leaf in respiration 
and into it in assimilation, whether this takes place by the stomatal 
openings or through the continuous surface of the cuticle, all possible 
extreme and intermediate views have been expressed in recent text- 
books of botany. On account of the smallness of the quantities of 
gas involved practically no attempt has hitherto been made to deter- 
mine this question by direct estimation. The existing experimental 
evidence is all of an indirect nature, and tends rather to support the 
view that the exchange is a cuticular phenomenon. 
An ingenious synthesis of Graham’s observations on the com- 
Abstract of a paper read before the Royal Society. 
