274 Notes on Recent Physiological Literature. 
seem to be required in places, like the extreme north, where the 
general intensity of the light is less. It therefore becomes interesting 
to know in absolute measure the Lichtgenuss of plants. The 
absolute Lightgenuss of an individual plant corresponds to the 
intensity, in Bunsen-Roscoe units, of the light falling on it at the 
brightest time of an average day during its active vegetative period. 
In practice an approximation to this is made by multiplying 
the Lichtgenuss fraction into the average mid-day light-intensity 
for that locality and that season of the year. In a region when this 
equalled 0 893 units, the Rel. L.R. given above for Poa annua would 
correspond to Abs. L.R.=0-893—0-297. 
Wiesner has in this way expressed the distribution of Poa 
annua at different times in early summer. This grass appears in 
more and more shaded spots as the year wears on. Thus near 
Vienna he found :— 
Early March, Rel. L.R., 1—^ and Abs. L.R., 0-89—0-29. 
Early April, „ 1—4 anc ^ » » 1*00—0-25. 
End of April, „ 1—and ,, „ 1*10—0 - 15. 
Middle of May „ 1—^ and „ „ 1-30—0-09. 
At Cairo, early in March, the Rel. L.R. of this grass is 1—but 
in March the sun at Cairo is as high as it is in Vienna at the 
middle of April when Rel. L.R.=J-—These higher minima in 
Vienna are attributed to the colder temperature prevailing at the 
corresponding time of year. 
Poa annua does not tolerate deep shade, but exhibits a high 
light-range. This is true of grasses in general, though, some shade- 
grasses extend into the shade of European woods where the light is 
only or 0-02 ; and into tropical woods to T J 0 =0-015. In the 
deepest shade in woods they are, however, absent. 
In considering the L.R. of trees we have the complication that 
the inner foliage of the crown is shaded by the outer. For ordinary 
trees the outer foliage is exposed to the full light of day. The 
illumination of the innermost leaves is measured by following down 
a branch into the crown until the innermost leaf is arrived at and 
here the sensitive paper is exposed. If the time taken to attain 
the standard grey is fifty-five times as long as when the paper is 
exposed on the free surface of the tree, then the Rel. L.R. of the 
tree is \— ft l 5 . The implication is that the tree could just live in 
such a light as sustains its most shaded leaves. 
Before all the leaves of the year are fully developed, higher 
minimum values are found, but in summer the minimum soon 
becomes stationary. 
