484 BOTANICAL GAZETTE [JUNE 
of the demands of plants for water, one of the most essential features 
of their environment. This unit was taken as grams per hour 
per 100 sq. cm. of leaf surface. 
2. While the use of rooted plants is most desirable, the results 
obtained through measuring the transpiration of cuttings, the 
stomates of which are closed at the time of collecting and during 
the period of adjustment to the apparatus, approach closely those 
of potted plants, except under extreme conditions of evaporating 
power of the air. Experiments run in duplicate are a check upon 
the behavior of individual specimens. 
3. Cuttings of hydrophytic plants wilt in a very short time in 
spite of every precaution, and experiments of any duration cannot 
be made upon them. Cuttings of other herbaceous plants, if 
properly cared for, may be used for at least 24 hours, and cuttings 
of shrubs and trees may be used for a still longer time. ; 
4. From the data obtained in this investigation, transpiration 
varies with the evaporating power of the air, that is, increases 
directly with increase of temperature, decrease of relative humidity, 
increase of air movement, and, other factors being equal, is greater 
in daylight than in darkness. The last factor, however, is due to 
the tendency toward increased internal temperature with absorp- 
tion of radiant energy. 
5. The transpiration of all plants experimented with was very 
low in winter, yet was demonstrable during the daytime of clear 
days, even at the lowest temperatures (—29°). Very frequently 
no transpiration could be detected during the night. The decided 
gain in weight which frequently occurred is to be attributed to the 
power possessed by the leaves of these evergreen ericads of absorb- 
ing water from a humid atmosphere. A gain always occurred on 
frosty nights and at times of high relative humidity. Both fresh 
and dried twigs of the evergreen ericads and of the conifers exhib- 
ited an ability to absorb water vapor from humid air. Such 
absorption is much greater in quantity per unit volume of leafy 
twig in the evergreen conifer Picea mariana than in the deciduous 
conifer Larix laricina, and in the scale-covered Chamaedaphne 
than in the other evergreen ericads. As such absorption not 
infrequently is three or four times as great as the transpiration 
