480 VENTILATING SYSTEM 



It is quite clear that the rate of transpiration of the same branch 

 varies considerably, according as the lenticels are open or closed. 

 Klebahn obtained concordant results with branches belonging to other 

 species of trees. 



As regards the distribution of lenticels, the author has found that 

 these structures are scattered uniformly over the surface in the case of 

 erect branches, while they tend to be aggregated upon the lower sides 

 of horizontal twigs ; but the asymmetrical distribution in the latter 

 instance becomes much less pronounced as the branches grow older. 

 Gleditschia triacantJios illustrates the dorsiventral arrangement in its 

 most extreme form ; in one set of estimations a piece of yearling twig 

 2 cm. in length was found to bear, on an average, 210 lenticels on the 

 lower side, as compared with 72 on the upper surface. In view of the 

 close physiological relation between stomata and lenticels, there can be 

 no doubt, that the preponderance of these pneumathodes on the lower 

 surfaces of the organs that bear them, is determined by the same con- 

 siderations in both cases. According to Devaux, lenticels also tend to 

 congregate in the vicinity of leaf-bases ; the same author states that 

 one or two lenticels invariably occur close to the insertion of adven- 

 titious roots. 



The so-called ventilating pits (Staubgriibchm), which are charac- 

 teristic of the stems and petioles of Cyatheaceae and Makattiaceae, 

 have certain features in common with lenticels. These organs have 

 recently been studied in .some detail by Hannig. 235 They consist of 

 rounded or oval depressions filled with a loose complementary tissue, 

 which is abundantly provided with intercellular spaces ; they arise 

 beneath stomata, and their development leads to the rupture of the 

 overlying portion of the epidermis. In all these points they closely 

 resemble ordinary lenticels. Although they are not provided with 

 a continuous meristem, the immediately adjoining cells of the paren- 

 chymatous ground tissue give rise to the complementary tissue by 

 irregular division. There can hardly be any doubt that these struc- 

 tures are really pneumathodes. In the Makattiaceae they only remain 

 functional as long as the leaf is not completely unfolded, and close as 

 soon as the lamina begins its photosynthetic activity. 



C. THE PNEUMATHODES OF AERIAL ROOTS. 



1. The pneumathodes of breathing-roots. 



It has already been explained (p. 44.3) that the breathing-roots of 

 Jussiaea ultimately throw off their epidermis, so that the aerenchyma 

 comes into direct contact with the surrounding medium. As a rule, 

 however, breathing roots are provided with special pneumathodes; in 



