272 Notes on Recent Physiological Literature. 
at 87', no value above 1*6 was recorded, but there the middle of 
the day was always somewhat clouded. 
Besides the intensity of the light, its duration is obviously of 
biological importance. Here latitude makes an enormous dif¬ 
ference, for during the vegetative period the sun is above the 
horizon for twenty-four hours in the arctic region, and only twelve 
in the tropics. Attempts are made to express this difference by 
computing daily “ light-sums 1 .” The highest actually observed in- 
Vienna during 1894 was 419 on June 17th, and the lowest 6'2 oir 
November 17th. It is not generally realised how much less the 
average daily light-sum is in winter than in summer. In January 
the average amount is 15-1 and in July 274-5. This means that if 
a plant could assimilate C0. 2 all day, and in proportion to the 
intensity of the illumination, it might accumulate nearly twenty 
times as much organic matter in a mid-summer day as in a mid¬ 
winter day. 
At Buitenzorg, although the sun rises higher in the heavens,, 
the actual observed light-sums were smaller than those of summer 
in Vienna, presumably due to the shorter day and more absorbing 
atmosphere. Thus Wiesner for the former place computed 252 iiv 
November and 227 in January. 
In Spitzbergen, in August, where the sun only rises to about 
27° above the horizon, the light-sum observed was 97, while at 
Vienna, on such winter days when the sun also only rises to 27°, the 
shorter day reduces the light-sum to 35—50. 
II. 
From this sketch of photo-chemical climate and the amount of 
light incident upon vegetation we may now turn to the individual 
plant and its illumination. 
The eye cannot be trusted to estimate the illumination of indi¬ 
vidual plants. A herb growing near a wall looks fully illuminated, 
but of course the wall must exclude nearly half the light of the sky, and 
measurement by exposing sensitive paper, shows that the intensity of 
illumination is about half that prevailing with a plant in the open. 
At the foot of a leafless tree in winter, even on the sunny side 
with the sun shining, there is a marked falling off in total illumination 
due to obstruction of much of the diffuse sky-light by the tree. 
On a sunny day in March, when the total light in the open was 
1 Comparisons of light-sums may be made by plotting the daily 
curve of intensity with hours as abscissae, and estimating the 
area enclosed between the curve and the base line. 
