314 PROTECTIVK AKHANGEMENTS OX THE EPIDERMIS. 



The importance of air-containing cells as a covering for succulent tissue must 

 also be considered in another i-elatiou. It is well known that evaporation from the 

 surface of fluid or a damp body is much increased by the warmth of the sun's rays. 

 On the other hand, if the heating is restricted, so also is the evaporation. If we use 

 a dry cloth to shade from the sun, we lower not only the temperature, but also the 

 amount of evaporation from the shaded body. The covering of air-containing hairs 

 on leaves may be compared to such dry screens, and its action may be demonstrated 

 by the following experiment : — Take two of the bi-coloured leaves of a Bramble 

 bush, which are smooth on the upper side, but covered with a white felt-work of 

 hairs on the lower, and whicli are exactly similar in size and position with regard to 

 the sun, being situated very near each other on the stem. If these leaves are 

 wrapped round thermometers, in such a w^ay that the leaf which covers one thermo- 

 meter bulb has its white felted side turned towards the sun, that covering the other, 

 the green hairless side, it will be found that the temperature in the leaf whose 

 smooth green side is directed towards the sun will in less than five minutes rise 

 2°-5° above that of the leaf whose white felted side is so directed. If such leaves 

 are plucked and exposed to the sun, some with the white felted side, others with the 

 smooth green side uppermost, the latter always shrivel and dry up much sooner than 

 the former. There can be no doubt, after this, that a diy coat of hair over succulent 

 plant tissue, which is exposed to the sun's rays, considerably restricts the heating of, 

 and exhalation from this tissue. 



The significance of the coverings of hair on portions of plants turned away fi'om 

 the sun, particularly on the under sides of flat and rolled leaves, has already been 

 discussed. These coverings are only of slight importance as a means of protection 

 against over-transpiration. In rare cases, indeed, it happens that the hairy covering 

 on the side of the leaf turned from the sun, the lining of the leaf, so to speak, must 

 act as a protection, since the flat leaf -lamina is so twisted and turned that the sun's 

 rays strike not on the upj)er but on the under surface. There are certain ferns of 

 Southern Europe (Geterach qßcinarum, Cheilanthes odora, Kotochlcena Marantce), 

 which, contrary to the habits of most of this shade-loving group, grow on blocks and 

 walls which are exposed to the burning sun. In these ferns the upper surface of 

 the leaf is sniooth, but the under, on the other hand, is thickly covered with dry 

 hair-scales. In wet weather the leaves are spread out flat, with the smooth 

 surface uppermost; in dry weather they become rolled up, and the under cottony 

 side is then exposed to the sun and to dry winds. Among the low herbaceous 

 growths of the Mediterranean flora, a like behaviour is shown by the W'idely distri- 

 buted Hawkweed, Hieraciuvi Pilosella, whose radical leaves, forming a rosette on 

 the soil, appear green on the upper and white on the under side, bj^ reason of a felt- 

 work of star-shaped hairs. In places where the ground easilj^ dries up, and when 

 there have been no show^ers for a long while, it is iisually seen that fii-st the margins 

 of the leaves turn up, and then by degrees the whole leaf becomes bent and rolled, 

 so that the lower side is turned towards the sun's raj's, ami the white felt of hairs 

 functions as a protective screen to the whole leaf. 



