392 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
while carbonic acid is exhaled through the membrane. (10) Humidity 
has a decided influence on the composition of the external membrane. 
(11) Slow or rapid desiccation diminishes the permeability, and leads to 
an accumulation of carbonic acid gas. (12) The variations in the 
pressure in the internal atmosphere are due to variations in the propor- 
tion of oxygen and carbonic acid. (13) A gaseous current through the 
pores is caused by differences between the external and internal pres- 
sures. (11) This current is one of the means by which nitrogen enters 
the internal tissues. (15) A constant purely passive circulation of 
nitrogen exists in all aerial plants. (16) Temperature modifies the com- 
position of the internal atmosphere. (17) Light also produces its 
effect. 
The author concludes by stating that the mechanism of exchanges 
can be classed under three headings: — (a) Effusion — Diffusion of free 
gas through the pores of the envelope under the influence of differences 
of pressure. ( b ) Dialysis — Diffusion of dissolved gas through the 
membrane of the envelope under the same influence. (c) Gaseous 
Current — General displacement of the total mass of mixed gases through 
the pores of the envelope under the influence of the difference of 
pressure which exists between the interior and exterior. 
(3) Irritability. 
Dependence of Sensitiveness on the Presence of Oxygen.* — Herr 
C. Correns has undertaken a series of experiments for the purpose of 
determining the correctness or otherwise of the ordinary view that the 
sensitiveness of organs is dependent on the presence of oxygen. Careful 
experiments made with a very exhaustive air-pump, aud with precautions 
against sources of error which are described in detail, brought out a 
source of error in the older observations, viz. that the exhaustion of the 
air causes at the same time, through increased transpiration, the rigid 
condition of the organs due to drought. 
The author classes movements of irritability under two heads, — 
those due to changes in the turgor only, and those due to growth, with 
or without previous change in turgor. Under the first head he investi- 
gated the movements of the leaves of Mimosa pudica, those of the 
stamens of Berberis and Helianthemum, of the stigma of Mimulus, of the 
filaments of Centaur ea, and the sleep-movements of the leaves of Oxalis 
and Leguminosm ; under the second head, sleep-movements of the same 
kind, viz. those of the tentacles of Drosera, of the tendrils of Passijlora, 
and movements of geotropism and heliotropism. 
1 he conclusion arrived at was that, at present, no general law can 
he stated for all these phenomena. The tentacles of Drosera are sensi- 
tive -when the minutest trace of oxygen is present ; while the tendrils 
of Passijlora require six per cent, of the normal amount of oxygen to 
bring out the sensitiveness to contact. With Mimosa the experiments 
were altogether indecisive ; and this was no doubt partly due to con- 
stitutional differences in the species operated upon ; since some seedlings, 
e. g. Helianthus, are geotropic with the least trace of oxygen, while 
others, e. g. Sinapis, only when a considerable proportion is present. 
* Flora, Ixxv. (1892) pp. 87-151. 
