64 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



pseudoplatanus have the wax coating only on the under side, which is 

 normally away from the sun; and many workers hare questioned the 

 drought-evading value of the wax in this location. Kerner, however, 

 has pointed out that even in a slight wind these leaves are turned 

 so that the under side is at least partly in the light, and he considers 

 that such coverings, even though on the under side, are of distinct 

 advantage. 



To show experimentally the value of the waxy covering in checking 

 transpiration, Tschirch (216) removed the wax from one of a pair of 

 leaves of Eucalyptus globulus and found that after 24 hours, the leaf 

 with the wax removed had lost 59.7 percent of the contained water, 

 while the control leaf with the wax still on it had lost only 42.2 percent. 



As has previously been seen, young leaves in which the cuticle is not 

 well developed transpire very freely; it is, therefore, not surprising 

 that buds are commonly protected against water loss by wax} T coatings, 

 and that the bud scales protect the tender leaves beneath from water 

 loss and other injuries has long been known. In the winter in Siberia, 

 in buds of woody plants (including Mains baccata Borkh., Betula alba 

 L.) the danger of drying out is overcome by a strong cutinization, 

 thick hairs, and waxy excretions of the bud scales, as pointed out by 

 Schostakowitsch (180). 



To test experimentally the protection provided by the bud scales 

 against transpiration in winter, Wiegand (234) cut off the scales from 

 buds of various trees (Pinus laricio, Aesculus hippocastanum) and a 

 part of the naked buds he then smeared with varnish. But as Burger- 

 stein {28, p. 197) has pointed out, it would have been much better to 

 have compared buds with and without their scales, especially if one 

 wishes to learn something about their behavior under natural condi- 

 tions. 



Hairs are found especially on broad-leaved evergreens and under 

 conditions such as those occurring in semiarid regions. According to 

 Kerner, there is not a single family of plants found in the Mediter- 

 ranean region which does not have somegenera whose leaves are pro- 

 vided with a heavy coating of hairs. Here, also, in many cases the 

 hairs are found chiefly on the under side, but the same argument 

 applies as in the case of waxy coatings, according to Kerner, who 

 states that the slightest movement of air turns these leaves toward 

 the light. In the opinion of the writer, however, Kerner has empha- 

 sized too much the light relation. In some of these instances the leaves 

 do not have to be turned in order for the hairs on the lower side to 

 serve as a protection against excessive transpiration, inasmuch as 

 they may protect against wind as well as against sunlight. Although 

 the importance of hairs in checking transpiration has probably been 

 somewhat exaggerated, as pointed out by Sayre (179), they are prob- 

 ably more of a protection against wind when on the lower side than 

 when on the upper side, which may explain their survival on leaves of 

 such trees as Populus alba L., Salix caprea L., Sorbus aria Crantz, 

 and Alnus rugosa Spreng. As pointed out above, hairs are also present 

 in buds, where they are sometimes strongly developed. In beech,, 

 sycamore, and other cases they may disappear later, being retained 

 only in the angles of the leaf veins. Brenner (21) found in various oaks 

 an increase in hairiness with an increase in insolation, which is in 

 agreement with the fact that in the mountains of central and southern 

 Europe, where an alpine vegetation is found, the number of forms with 



