64 Ricuarbs AND MacDouGAL: INFLUENCE OF 
go per cent. of carbon monoxide, practical asphyxiation occurred 
in Meibomia, Haematoxylon and Mimosa, the leaves becoming dis- 
colored and turning brown, but not being cast away, the small 
amount of oxygen present and the strong toxic action of the other 
gases cooperating to kill the leaf before separatory layers could 
be formed. | 
In all tests in which a supply of oxygen fairly equal to the 
normal was present, the leaves of the three plants were discolored 
and cast away within a week, and at the end of that period appeared 
to be much more seriously damaged than those in higher percent- 
ages, which appearance however might be seen to be erroneous 
on closer examination. Plants exposed to both higher and lower 
proportions for two weeks revived and sent out new leaves when 
placed in the propagating houses. Tests with young plants of 
Opuntia tuna and with cuttings of a species of Mesembryanthemum ~ 
showed that the last named is much more readily killed by the 
gases, the entire plant perishing after exposures for a month to 
atmospheres containing as much as 25.5 per cent. carbon mon- 
oxde. 
In marked contrast with the foregoing is the behavior of the 
gametophytes of some of the mosses. Such forms as Catherinea 
angustata, Dicranella heterophylla and Physcomitrium turbinatum, 
were able to endure atmospheres containing the highest practic- 
able proportions of carbon monoxide or of illuminating gas for 
periods of over three months, with but little damage and that to 
the older leaves, in which the cell-contents and chloroplasts were 
variously and deleteriously affected. In two instances sporophytes 
were formed. During this period the gametophytes had formed 
numbers of new leaves, and their resistance could not be attri- 
buted to the possibility of being in a resting condition. 
Some observations on a more delicate moss, presumably Mnium 
undulatum, show that these gases are not always without effect 
even on these plants. It should be said, however, that the 
changes in the cell-contents herein noted proceed so far and no 
farther, without involving the death of the plant as a whole. 
The JZium leaves after exposure for about three weeks to at- 
mospheres of 80 per cent. CO and of 80 per cent. gas respectively, 
were in general lighter in color. Microscopical examination con- — 
