432 BOTANICAL GAZETTE [JUNE 
covering of certain species of Quercus became thicker when exposed 
to greater intensity of sunlight. : 
HABERLANDT’ selected two nearly equal leaves of the same oppo- 
site pair from a plant of Stachys lanata. The under side of each was 
coated with cocoa butter, and from the upper side of one the hair 
was carefully removed with curved scissors. The two petioles were 
placed in dishes containing water, and the leaves were left to transpire 
24 hours at 20 to 25° C. in the shade. The hairy leaf lost 0.646%" 
of water and the hairless one 0.915%", which was therefore in the 
ratio of 1:1.42. The leaves were then exposed to intermittent 
sunshine for one hour; the hairy leaf now lost o.08¢™ and the hairless 
one 0.167£™, or in the ratio 1:2.09. He concludes therefore that 
the hairy covering is especially important in restricting the transpira- 
tion in sunlight, but that it also operates in a less degree in diffuse 
light. In the sunshine it probably prevents the extreme heating of 
the leaf, in diffuse light it retards the diffusion of the air. The error 
that might be expected from the exposure of uncutinized surfaces, 
when the hairs are cross-sectioned, HABERLANDT found by mathe- 
matical calculation to be ins gnificant. 
BAuMERT’ tested the hea screening power of hairy coverings 
and found that in one case ¢ leaf deprived of hair became 37-5 
per cent warmer than a normal one. 
It was soon found that to obtain experimental data regarding the 
effectiveness of various coverings was not easy. In fact, it seems 
almost impossible to obtain accurate and detailed results because 
of the many factors that enter. No two leaves are exactly alike in 
size, or in thickness of cutin, or in water content; and it is very diffi- 
cult to remove the hairy covering for comparative experiments with- 
out injuring the tissue of the leaf. Moreover, the stomates may be 
open at times and closed at others, and the two leaves used may not 
be alike in this respect. 
Fortunately there seemed to be a method of approaching the prob- 
lem in an indirect way. It is quite generally recognized that by far 
© HABERLANDT, G., Physiologische Pflanzenanatomie. 3. Aufl. 116. 1904 and 
possibly in earlier editions. 
7 BauMERT, K., Experimentelle Untersuchungen iiber Lichtschutzeinrichtunge™ _ 
an griinen Blattern. Beitr. Biol. Pflanz. 9:83-162. 1907; and Inaug. Diss. Erlangen- 
1907. 
