EPIDERMIS. 47 



base of the petiole of the Ferns (comp. below, Chap. IX). The occurrence of 

 stomata in hollow depressions on the under surface of the leaf of many species of 

 Banksia, and Dryandra ^ is a special case of the same thing, which derives its peculiar 

 appearance only from the strong outgrowth of the nerves on the under side of the 

 leaf On the under surface of the leaf of Nerium oleander there alternate, between 

 the nerves, spots with and without stomata. The latter occur in depressions of the 

 leaf-surface, which are deep and narrow-necked, and covered thickly with hairs ". 



On the part or band, which bears stomata, the air-pores are besides in rare 

 cases limited to circumscribed spots, separated by intermediate areas without stomata : 

 the spots are then also characterised by a special form of the epidermal cells. Thus 

 on the flat underside of the leaf of Saxifraga sarmentosa numerous stomata are 

 collected in circular groups, removed some distance from one another^; on the under 

 side of the leaf of many (but not all) Begonias, e.g. B. manicata, spathulata, Dr^gei, 

 heracleifolia, two to six or more stomata stand side by side over a great common 

 respiratory cavity *. 



As a rule there is over large surfaces and bands an almost uniform distribution 

 of air-pores. Their number, both relatively to the number of the epidermal cells and 

 to a definite superficial space, varies within wide limits according to the organ and 

 species, and to some extent according to the condition of the surrounding medium. 

 In the first relation we have, on the one hand, one stoma to almost every epidermal 

 cell, e.g. in leaves of Monocotyledons, as Iris ; on the other hand, as in the stems of 

 many woody plants, Cuscuta, &c., there is one stoma to many hundred epidermal 

 cells'*. In the other relation, the maximum numbers found for i^i^n were 625 

 (under surface of leaf of Olea Europsea ''), and 7x6 (under surface of the leaf of 

 Brassica Rapa''). For most foliage leaves the number lies between 40 and 300, 

 rarely higher or lower ^ As above stated, on the stems of many woody plants the 

 stomata lie several millemeters, or still further apart, as is conspicuously shown on 

 the formation of lenticels of Sambucus, Acer, &c. (Chap. XV). A like stage of de- 

 velopment being of course assumed, there may be laid down for each part of each 

 species a definite average number, which is, it is true, liable to not inconsiderable 

 individual variations. Karelstschikoff communicates examples of individual variation. 

 On an equal surface (the same field of the microscope, which was not measured) 

 six leaves of Viola tricolor, each taken from a different stock, had on the under 

 surface between 21 and 43, the majority between 30 and 40; on the upper side 

 o to 14, the majority between 9 and 13. 



* Von Mohl, Spaltoffn. d. Proteaceen, Verm. Schriften, p. 245. 



^ Amici, Ann. Sci. Nat. XXI. p. 438.— H. Krocker, I.e. p. 13. — Meyen, Physiol. I. p. 291. — 

 Compare Pfitzer, Pringsheim's Jahrb. VIII. p. 49. 

 ^ Treviranus, Verm. Schriften, IV. 30. 



* Viviani, Delia struttura degli organ, element, torn. I. fig. 4, p. 151, quoted by Treviranus, 

 Physiol. 1. p. 466. — H. Krocker, I.e. p. 13, fig. 39. — Meyen, Physiol, I. p. 280, Tab. V. — On the 

 development of the groups, compare Pfitzer, Pringsh. Jahrb. VII. p. 551. 



^ Compare the figures of Strasburger, Pringsheim's Jahrb. V; Hildebrand, Botan. Zeitg. 1870, 

 Taf. I. 



^ Weiss, Unters. iiber die Zahlen- und Grossenverhaltn. d. Spaltoffnitngen. Pringsheim's Jahrb. 

 IV. p. I24fir. 



' linger, Anatom. und Physiol, p. 193. * Compare Weiss, I.e. 



