RECEPTACLES FOR EXCRETIONS., 241 
lateral cells (Fig. 155 n). The number of the stomata varies 
exceedingly. Upon a square millimeter of surface of a leaf 
from ten to eight hundred of them may be present. 
The distribution of the stomata, as also the shape and size of 
the epidermal cells, is employed by Bell’ as a means of dis- 
tinguishing tea, elder, willow and sloe, or black-thorn leaves. 
The periderm, like the epidermis, is also occasionally traversed 
by open gas passages. These are the lenticels ’ or bark-pores. 
They are mostly produced beneath the stomata, and, as they 
consist of roundish, suberized cells, they form small, sharply 
circumscribed corky protuberances, which assume on the older 
organs the function of the stomata, that is, they regulate the 
interchange of gases. For some barks the lenticels are charac- 
teristic (Rhamnus Frangulu, Solanum Duleamara). Such 
corky growths occur also upon leaves (Lucalyptus, Fig. 128 hk, 
Dammara).* 
8. Receptacles for Hzcretions. 
During the change of matter incidental to the life of the 
plant, there are formed certain substances which, so far as our 
knowledge extends, find no further application, and are therefore 
termed excretions.‘ 
They may either be produced in more or less peculiarly shaped 
cells or cellular passages (oil-cells, mucilage-cells, latex-tubes), 
1 « Die Analyse der Nahrungsmittel,” i. (Berlin, 1882), p. 36. 
° Diminutive of the Latin Jens, a lens, on account of their form. Re- 
garding to the lenticels, compare Stahl, Botan. Zeit., 1873, No. 36. 
Klebahn, Ber. d. Deutsch. botan. Ges., 1833, and ‘‘ Die Rindenporen. 
Ein Beitrag zur Kenntniss des Baues und der Function der Lenticellen 
und der analogen Rindenbildungen,” Jena, 1884, where the literature is 
also given. ie ie 
’ Bachmann, ‘“‘ Ueber Korkwucherungen auf Blattern,” Pringsheim’s 
Jahrb., xii., p. 190. ; : 
‘Regarding the limitation of the terms excretion and secretion, 
compare the interesting investigation of Szyszylowicz, “ Ueber die 
Sekretbehialterfder flichtigen Oele (im Pflanzenreich.” Denkwiirdig- 
keiten der Krakauer Akademie, 1880 (Bot. Centralbl., viii., 1881, p. 259). 
16 
