34 CELLULAR TISSUE. 



Carica, laurifolia, Neumanni, iiymphaeifolia, australfs, lutescens, salicifolia ^ Its thickness 

 varies according to the species, and is on an average less on the under surface than on the 

 upper. Certain individual cells of the original epidermis remain undivided, and grow to form 

 the sac-shaped cystolith-cells, which project deep into the inner tissue of the leaf (§21). 

 Ficus lutescens and F. ulmifolia have on the upper surface of the leaf an epidermis of 

 two or three layers, on the under surface of only one layer (Schacht. I.e. p. 142, Fig. 10). 



A many-layered epidermis has further been described by Nicolai ^ and Pfitzer (/. c.) in 

 the roots of Crinum bracteatum and C. americanum. 



Lastly, it occurs closely connected with hair structures, on many glandular spots, to be 

 described later,*.^. in Passiflora, on the ends of the leaf-teeth of Drosera,etc. Comp. § 1 8, 20. 



Sect. 5. Stomata^ (comp. Fig. 10-18). Between the cells, of the epidermis 

 there lie definitely distributed pairs of cells, whose sides opposed to one another are 

 concave, and between these a slit is left open. The slit extends through the whole 

 height of the epidermis, forming an open communication between the surrounding 

 medium and an intercellular space inside it, which is called the respiratory cavity 

 (Athemhohle) *. The apparatus consisting of the pair of cells with the slit is called a 

 pore or stoma'^ {Spaltoffnutig, Porus, stoma), and the cells bordering on the slit 

 siomatal-, pore- or guard-cells. 



The general form of the mature stoma is in surface view (with medium 

 turgescence) usually nearly elliptical; rarely relatively narrow, usually widely-elliptical 

 (in 162 out of 174 cases observed by Weiss) ; further it is in some few cases almost 

 circular *, the special forms being endlessly various according to the species. The 

 irregular three- to four-cornered stomata in Salvinia and AzoUa ' form a remarkable 

 exception to the rule : each guard-cell corresponds to one half (in case of the usual 

 elliptical form, a longitudinal half) of the whole form ; both are, under medium 

 turgescence, curved in a half-moon or sausage-shape ; they are directly connected 

 by their ends, and by the ends and the convex sides they are joined uninterruptedly 

 with the surrounding epidermal cells. The concave sides are turned towards 

 one another and bound the slit, which is usually elongated in the direction of the 

 division-wall by which the ends of the guard-cells touch one another ; in Azolla on 

 the contrary (Slrasbi^rger, l.c>) at right angles to this direction. The transverse 

 section of the guard-cell (Figs. 10, 11) is generally round or forms an ellipse 



' Meyen, Phytotoraie, p. 311.— Miiller's Archiv. 1839, p. 264.— Payen, Mem. pr&ent. 4.1'acad. 

 Bes Sciences, torn. IX. — Schacht, Abhandl. Senckenb. Gesellsch. I. — Unger, Anatomie u. Physiol. 



p. 190.' — Hofmeister, Pflanzenzelle, p. 180.— Weddell, Ann. des Sci. Nat.' 4 s^r. torn. II. p. 271. 



Pfitzer, /. c. p. 35. 



^ Schriften der Physic. CEcon. Gesellsch. z. Konigsberg, VI. p. 73. 



= [Cf. further, L. Reinhardt, Einige Mitt. ii. 4. Eutw. d. Spalt., in Russian, Ref. Bot. Jahresber. 

 1879, p. 30.— Schwendener, Botan. Zeitg. 1882, p. 233.— Tschirsch, Beitr. z. vergl. Anat. d. Spalt. Ap- 

 parats. Verhandl. Bot. Ver. Prov. Brandenburg, Ref. Bot. Centralbl. 1881. Bd. VI. p. 341. Sachs, 

 Vorlesungen, 1882, p. 395.] 



• Unger, Exantheme d. Pfl. p. 43. 



= Spaltoffnung, Sprengel, Anleitg. z. Kenntniss. d. Gewachse ; Bau und Natur d. Gewachse, 

 p. 180. Poren, Hedwig, Zerstr. Abhandl. p. 1 16 ; Rudolphi, Moldenhawer. Stomata, De CandoUe, 

 Organograph. vegetale, I. p. 78. StomaHa, Link, Grundlehren, p. 108. The name dermal glands 

 (Hautdrusen), later resumed by Link and Meyen, has hardly any further historical interest.— For the 

 history of these parts, so often mentioned since Malpighi and Grew (Anatomy of Plants, pi. XLVIIl), 

 compare Treviranus, Physiol. I. p. 462 ; Meyen, Phytotomie, p. 97 ; Pflanzenphysiol. I. p. 271. 



" For details compare A. Weiss, in Pringsheim's Jahrb. IV. p. 123, &c. 



' Compare Strasburger, Pringsheim's Jahrb. V. Taf. 36 : idem, Ue'ber Azolla, Taf. III. 



