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1907] CURRENT LITERATURE 217 
and rapidly along rock faces, and its real evaporating surface soon lies below the 
actual surface, the upper dry layers acting like a dust mulch. The seedlings send 
roots downward very rapidly and are slow in developing foliage until the root 
system is extended deep. The sap of cacti is found to have an osmotic pressure 
not notably higher than that of many mesophytes; but the gums, etc., in their 
cells, which do not sensibly raise osmotic pressure, hold back water. An evapo- 
rimeter, consisting of an exposed tube of porous porcelain filled with water and 
connected with a burette, enables ready measurement or automatic record of the 
effect of atmospheric conditions on evaporation, and by calibration the measure- 
ments may be converted into evaporation rate per surface unit of free water. 
Experiments show that no clue to real facts is given a study of transpiration from 
plants in a closed chamber,’ because of the striking effects of air currents. Rela- 
tive transpiration, i. e., the ratio of evaporation from a plant to evaporation from 
the same surface of free water, is proposed as the best way of expressing the 
facts. The highest relative evaporation noted was 0.785 and the lowest 0.008. 
Observations indicate a ‘physiological regulation” of transpiration, in which 
air temperature is apparently the dominant factor. 
Throughout, the work shows the purpose of the author to grapple with the 
problem as a physical one; only here and there a phrase survives, such as “‘the 
absorptive power exerted by the plant,” that recalls the less modern attitude. 
Surely it is only by conceiving the plant, the air, and the soil as a system, within 
which exchanges occur under very definite and definable physical conditions, 
that we can hope to state these exchanges intelligibly. Why not modify the 
evaporimeter by putting the foot of its water column into the soil and then try 
to state the changes within that system? When that can be done, there is hope 
for plant evaporation, and not till then —C. R. B 
MINOR NOTICES 
Key to woody plants.—In 1904 WIEGAND and Foxwortay published a very 
useful Key to the genera of woody plants in winter, including the genera with 
hardy aa — ave cee or in si memes _ the state of 
New York. Th appeared.® 
The text has been revised, but not much paste two or three genera have 
been added, and the keys to the conifers have been considerably expanded.— 
Be 
Food for plants.—A new edition of a booklet under this title by HARRIS and 
Myers is edited and published by WM. S. Myers, who is now devoting his time 
and energy to the nitrate of soda propaganda. It goes without saying that the 
term “food” as used applies to the mineral salts which yield nitrogen and phos- 
phorus. The book is a queer mixture of general information for the curious, 
7 id CANNon’s polymeter method, Bull. Torr. Bot. Club 32:575- 1905. 
EGAND, K. M., and Foxwortny, F. W., A key to the genera of = plants 
in oe Second edition. pp. 33. Ithaca, N. Y.: The authors. 25 ce 
