138 DERMAL SYSTEM 



the Pear, twenty-five-fold in the Elder, thirty-six-fold in the Lilac, and 

 forty-five-fold in the Horse-chestnut. 



The fact that a young cork-film in general restricts transpiration less 

 effectually than a well-developed epidermis is very easily accounted for ; 

 an astomatic epidermis is absolutely continuous, wdiereas, owing to the 

 secondary growth in thickness of the branch, a two- or three-year-old 

 cork-film is traversed by fine radial cracks, the effect of which in 

 augmenting transpiration cannot be eliminated in experiments such as 

 those described above. This conclusion is confirmed by the fact 

 established by Wiesner and Pacher, 80 that two- and three-year-old 

 twigs of Horse-chestnut transpire more vigorously than yearling 

 branches ; from the fourth year onwards, however, a marked improve- 

 ment is noticeable as regards the control of transpiration, a circumstance 

 which must be attributed to the steady increase in the thickness of the 

 corky integument. 



In any case, corky tissues constitute, even in thin layers, a very 

 effective instrument for the reduction of transpiration. Those brandies, 

 twigs, and buds of woody plants which have to live through the winter 

 certainly require this protection, if only because transpiration goes on 

 slowly even at temperatures below zero. Mention may be made of 

 certain experiments, performed by Wiesner and Pacher, which bear 

 upon this point. A two-year-old Horse-chestnut twig set up in a 

 special transpiration apparatus lost "323 per cent, of its weight after 

 transpiring for twenty-four hours at temperatures ranging from 

 -3-5C. C to -10-5 C; between - 5'5 C. and -13C. the same 

 branch lost *1 99 per cent, of water in the same time. Similarly, a 

 three-year-old Oak twig lost by transpiration in twenty-four hours 

 251 per cent, by weight between 3 C. and 8*5 C, and 192 

 per cent, between 5"5C. and 13 C. The transpiratory activity 

 of branches is thus by no means negligible, even at such low tem- 

 peratures, and it is probable that this statement is equally applicable 

 to winter buds, especially as these structures are known to suffer very 

 considerable losses by transpiration at somewhat higher temperatures. 

 Thus Wiesner and Pacher estimated the daily loss by transpiration at 

 about L523 1'6 per cent, in the case of a winter bud of Horse- 

 chestnut. If, therefore, twigs and branches require to be protected 

 against desiccation, the same necessity will arise in the case of winter- 

 buds : as a matter of fact, bud-scales very often are provided witli a 

 layer of cork, which is situated beneath the outer epidermis 81 (in the 

 case of Aesculus the epidermis is cast when the cork appears). 



( !ork is very impervious to gases. Careful experiments carried out 

 by Wiesner 82 have shown that quite thin plates of cork obtained from 

 the Cork-Oak or from Potato tubers are practically impervious to 



