1900] | BRIEFER ARTICLES 349 
Jan. 9, Jan. 10, Jan. 11. Jan, 12, Jan. 13, Jan. 15, Jan. 17, Jan, 20, 
IPM, 3P.M. 4P.M. 3PM. 3P.M. 4PM. 3 P.M, 
I P.M. P.M 
Ficus (immature) 217 26™™ 35™™ 42™™ 46™ 58" 665" 7458 
Begonia: smooth, green 18 = 30 36 37 43 44 
Begonia: red 31 ruptured 
Hedera 27 ked 
Cyclamen 38 61 go 110. .120 140. 458. ;7 484 
Farfugum 56 70 o8 . 118 t46 2677. re 
34 
“Croton” (Codiaeum) 65 123 153 168 177 184 185 184 
The see-sawing, especially evident with the Cyclamen and Farfu- 
gum leaves, is a mystery. From other experiments I know that diffu- 
sion through the Hedera leaf is considerably faster than through 
mature leaves of Ficus. Nerium leaves are also among the more 
resistant. The resistance to diffusion increases with the age of the 
leaf and with its dryness. According to Detmer dry Hedera leaves 
are impermeable ; but air-dry and entirely brown leaves of Ficus still 
permit a slow escape of CO,. From these and a few other experi- 
ments I regard the Croton* leaf as the best adapted of the leaves of 
ordinary greenhouse plants for the demonstration within a reasonable 
time of the diffusion of gases through the cuticle. 
What the actual process of the passage of any gas through the cuticle 
is would be hard to determine. Of course it is not mere diffusion, else 
the oxygen and nitrogen of the atmosphere would pass more rapidly 
than the heavier carbonic acid. It may be that they dissolve in the cuti- 
cle, the carbon dioxide much more readily : or possibly they enter into 
Some loose chemical combination, as oxygen is taken up and given off 
by haemoglobin. However that is, the fact that it was under the 
Same leaf that the mercury rose fastest, and began to fall first is prob- 
ably due to its thin cuticle, so that any gas must traverse it but a short 
distance. And in any case the external factors governing the rate of 
passage would be the same. 
If it is desired merely to show how fast carbon dioxide can pass 
through a plant membrane, without concern as to the immediate appli- 
cation of the experiment to ordinary plant problems, one of the water 
lilies is a still better subject. It should be fixed with the under sur- 
face exposed to the air. With all other conditions the same as in the 
other experiments, and with part of a leaf of Mymphaa alba as the 
Closing membrane, the mercury rose 185™" in four hours! In three 
days it passed 10 =. 
“The plants commonly known in greenhouses as Croton are really Codizeum. 
