348 BOTANICAL GAZETTE [may 
middle. On the smooth surfaces, where the cotyledons meet, the melted 
area is even in outline and easy to see. Average dimensions of ten 
such melted areas were: along axis D, 0.0649 inch; along D’, 
0.0714 inch. The diameters of the cells which conduct the heat are 
about equal in these two directions. The cells are elongated along the 
axis D", but the cotyledons cannot be cut so as to give a surface 
smooth enough to work with. The result—conduction more difficult 
in the direction of greater swelling — harmonizes with the hypothesis. 
But it is hard to imagine an arrangement of the micellae by which 
their long axes are, for instance, parallel in the entire cellular struc- 
ture, without regard to the individual cells. 
2. Gas diffusion through the cuticle. 
Experiments on the diffusion of carbon dioxide through the cuticle 
are interesting in relation to the gas nutrition of the plant, and have 
an even greater value as aids in the explanation of the most funda- 
mental physical processes. For we are almost obliged to preface our 
treatment of osmosis with a rehearsal of the mechanical theory of 
gases : and then we are too apt to introduce into the particular expla- 
nation of osmosis a new element, in the semi-permeable membrane. 
The plant cuticle is an appropriate instance of a membrane nearly 
semi-permeable to gases. With the leaves usually employed the 
experiment is very slow, and I have tested a number of leaves in search 
of some one with which the course of the experiment would be enough 
more rapid to recommend it for class demonstration. 
All the leaves in the following lot have the stomata confined to 
the lower surface, and are available for winter use. They were set up 
in the usual way: a cork fitted over one end of a glass tube, and 
heated in paraffin, and the leaf sealed over the hole in the middle of 
the cork. On each leaf all except an area of 121°°"™ over the hole 
was covered with paraffin. Of course it was necessary to have the 
upper surface of the leaf exposed, instead of placing it next the 
cork, as Detmer recommends for single experiments. The tubes, . 
21™ in length, were then filled with mercury, which was displaced in 
a mercury bath with carbonic acid. As only a comparison was sought, 
barometric readings were neglected: the temperature was reasonably 
constant. All were in the same mercury bath. The experiment bega? 
January 8. The height of the mercury column under the various 
leaves was as follows: 
