160 BOTANICAL GAZETTE [FEBRUARY 
At the time of separation of the mother cells, the growth of the capsule is 
checked, while the a continues growth, leaving quite a space between 
capsule and cal The capsule and seta soon resume growth, fill the 
cavity, and eed “* calyptra. No pseudoperianth, such as is found in 
Marchantia, is present. A sheath, which is a specialized portion of the gameto- 
phore, invests the calyptra—W. J. G. Lanp. 
Evaporation measurements.—The porous cup atmometer is now recognized 
by ecologists as one of the best instruments for measuring the evaporating 
power of the air, which is perhaps the most important climatic factor, or set 
of factors, in determining the vegetation of any locality. One difficulty in 
its operation has been that rain falling upon the exposed cup penetrates to 
some extent into the reservoir and vitiates the readings immediately following. 
To obviate this difficulty Lrvincston* has devised a rain-correcting atmometer 
with a mercury valve preventing any water from entering the reservoir. He 
also emphasizes* the importance of using nothing but the purest distilled water 
in the instrument and of standardizing the cups at frequent intervals. Recog- 
nizing the necessity of some uniform unit of standardization, in order that 
the results of the various workers may be comparable, he proposes that the 
standard cup be one that loses water at the same rate as 45 sq. cm. of water 
surface exposed in a Petri dish 1.5 cm. high and kept constantly filled to the 
depth of 3 mm. Microorganisms in the cups may be prevented by rinsing 
the cups and reservoirs with weak mercuric chlorid solution. It has also been 
found desirable to operate two or more cups at each station, as accidents are 
not likely to occur cp tre to all, and thus an unbroken record is bo 
more probable.—Geo. D. Fut 
* Livineston, B. E., A rain-correcting atmometer for ecological purposes. Plant 
World 13:79-82. 19 
42 
Igto, 
’ etal of the porous cup atmometer. Plant World 13:11I-119- 
