1915] CURRENT LITERATURE 331 
applies to photo-perception, with a coefficient of about 2.6 for 10° C. rise in 
temperature up to 30° C. It also applies to photo-reaction with a considerably 
ro 
perception rate, indicating that the process is a strictly photo-chemical reaction 
such as occurs on the photographic plate. D£EVrreEs’ work, on the other hand, 
lines these two processes up, so far as they are influenced in rate by tempera- 
ture, with chemical reactions in homogeneous solutions in general, and with 
photosynthesis, se ola: and geo-perception.”7 From o° to 25° C. the per- 
ception speed was cseiaaaries of the time of previous warming. Long pre- 
vious warming at 27.5-30° C. hastened perception rate, and such previous 
heating at 32.5° C. or higher temperatures slowed the perception rate. One 
hour’s heating at 39° C. lowered the perception speed at 20° C. more than four- 
fold. This effect entirely disappeared, however, after four hours’ storage at 
20° C., and is therefore considered rather a matter of hysteresis than of the 
accumulation of poisonous materials —WILLIAM CROCKER. 
Invasion of a prairie grove.—In the high prairie just outside of Lincoln, 
Nebraska, a grove was started about forty years ago by running furrows at 
intervals of 4-6 feet through the prairie and dropping the tree seeds into the 
furrows. At present about 20 acres are thus forested with Fraxinus penn- 
sylvanica, Juglans nigra, Ulmus fulva, Acer saccharinum, and A. Negundo. 
No culture has been attempted at any time during the history of the grove, 
nor has there been any damage by fire or grazing, hence the forested area 
affords an exceptionally good demonstration of the fact that trees grow freely 
once they are planted in this prairie soil, although they almost never invade 
the grasslands, and it also provides an unusually good opportunity of studying 
the changes in the undergrowth ree resulting from the changed con- 
ditions due to the tree growth. Poot in investigating the character of the 
invasions has found that not only has the prairie sod gone, but nearly every 
one of the original prairie species has entirely disappeared, being replaced by 
some go invading species, of nia 85 per cent are mesophytic and 60 per cent 
are distinctly woodland. sts of these species prove how completely the 
area has been transformed a prairie to forest ina very short period. Doubt- 
less changes in soil moisture, evaporation intensity, and light as the trees devel- 
oped led to the changes in the undergrowth. Poot has these and other factors 
under investigation and doubtless his results will form a valuable contribution 
to the understanding of the problems of the relations existing between the 
grasslands and the forests —Gro. D. FULLER. 
7 Bor. Gaz. §0:233-234. 1910; 51:230. 
dere hs J., The invasion of a aur a ee grove. Proc. Soc. Amer. For. 
10:1-8. 
