— 
ee hn 
When I first observed this phenomenon I was surprised that the oldest 
leaves showed the most active transpiration, but a few weeks later a paper 
by Bergen ** was received showing that the coconut is not peculiar in this 
respect. ‘The figures given above are not corrected for area; if this were 
done it would still further emphasize the difference, because in both cases 
the oldest leaves had the least exposed area. The relative activity of cut 
pinne and those in situ was at first exactly the same in all cases and so 
demanded no correction, but the oldest leaves were always the last to show 
an extreme depression. The transpiration from the upper surface was 
slightly more active in the latter, but it was not enough so to account for 
any great proportion of the extra quantity. A considerable part of the 
total area of the oldest leaves was occupied by small, scattered brown 
spots, and the leaf was dead two months after these observations. The 
tree was a young one. 
Thirty or more determinations of the transpiration during the night 
have all shown concordant results, the rate being about 1 per cent or even 
less of the greatest during the day; the total transpiration for an entire 
night was about one-tenth of that during one hour of sunlight at midday. 
Three factors are responsible for this great nocturnal depression—the 
darkness, the lower temperature, and the higher relative humidity. The 
complete experimental analysis of these three factors was practically 
impossible, but the cobalt test, being independent of the moisture of the 
environment, is capable of showing the inflence of the illumination inde- 
pendently of the relative humidity. 
By this test it has repeatedly been proven that a very slight shade will 
to a certain extent almost immediately depress the transpiration. Of 
course, actual darkness has a very much greater effect. In using the 
cobalt test T held the glass slides to the leaf with cork clamps, and there- 
fore the spot immediately between these was in approximate darkness. 
When, in the first test, the paper reddened in about four minutes, the 
change was not appreciably hindered by the cork; but if this first test 
required more time, and always during subsequent tests, the darkened 
part of the paper was very evidently slower in turning. Beginning at 
9.18 a. m. January 21 the intervals required for reddening were ten 
minutes and nine minutes; then, with a light haze over the sun, fifteen 
minutes; all these for the illuminated part of the paper. After the last 
determination, the paper was left until the part under the cork was red- 
dened; in about forty minutes water was precipitating on the glass over 
the lighted leaf, but the darkened paper was still bluish, only becoming as 
red as the standard after ninety-five minutes—that is, it took more than 
six times as long to change as it did when a mere haze weakened the light. 
~ Bergen, J. Y.: Relative Transpiration of Old and New Leaves of the Myrtus 
Type, Bot. Gaz. (1904), 38, 446. “The leaves of six out of the eight species 
studied transpire more for equal areas when fifteen to eighteen months old than 
they do when they have just reached their maximum area (i. e., at three or four 
months) .” 
