EPIDEkMIS. 



45 



tangential divisions accompany the extension of the epidermal' cells perpendicularly to 

 the surface, either successively from within outwards (Ficus), or the converse (Begonia, 

 Peperomia). The extension and division of the epidermal cells above mentioned always 

 begins after the differentiation of the initial cells of the stomata. The stoma itself re- 

 mains, so to speak, always a single layer, the same is the case with the cells immediately 

 surrounding it (subsidiary cells) in Begonia', but in Ficus, tangential divisions appear 

 also in the subsidiary cells, which arise according to the type (2, a) ; hence a ring of sub- 

 sidiary cells two or three layers deep''. In the growth of the epidermis perpendicular 

 to the surface this difference here occurs, that single stomata fully developed at first, 

 before the tangential division begins, remain at the surface ; the majority are matured 

 later, and are deeply depressed below it (comp. Fig. 18). The superficial stomata first 

 developed are surrounded by several partitioned zones of subsidiary cells. 



For the relative position of the stomata the rule holds, that in elongated parts all 

 the slits run parallel to the longitudinal axis. In parts which do not grow specially 

 in length the slits are arranged apparently without rule in different directions. 

 Exceptions to this rule are the stomata on the stems of Viscum album ', Cassytha, 

 Thesium, Choretrum, Mida, Myoschilus, Anthobolus, Exocarpus, Arceuthobium, 

 Antidaphne, Loranthus, Lepidoceras, Nuytsia*, Colletia^, Santalum album, Sali- 

 cornia^ Casuarina', Staphylea pinnata, on the under, side of the leaf of Philesia 

 buxifolia. Here the slits run perpendicular to the axis of the whole organ, often 

 (e.g. Salicornia, Arceuthobium, CoUetia, Philesia) the epidermal cells are at the 

 same time extended transversely. 



Sect. 6. According to special differences of form, structure, and arrangement, 

 two varieties of stoma may be distinguished, which may briefly be termed air^pores 

 (or stomata), and water-pores. Both may occur separately, or side by side in one 

 piece of Epidermis. 



Sect. 7. The air-pores show the slit itself, during normal vegetation, filled 

 with air ; they lead from the surrounding medium directly into the respiratory cavity, 

 which is also filled with air. Its guard-cells are, with exception of the abnormal 

 case of Kaulfussia, we may say always capable of change of curvature, and the slit 

 therefore of variable dilatation. They represent accordingly to a certain extent 

 openings in the epidermis which are capable of closing, through which the air 

 enclosed in the plant communicates with that surrounding it. Their arrangement, 

 presence, and absence are thereby generally defined- 



Air-pores, and all stomata whatever, are completely absent in roots. Of 

 the other parts of the plant hardly one can be named on which they may not, at 

 least in many cases, be observed '. 



' Pfitzer, Ueber d. Mehrschichtige Epidermis, &c., Pringsheim's Jahrb. VIII. /. c. 



^ Strasburger, I.e. Tab. 41, figs. 1.^5-138, and our fig. 18, both oi Ficus elastica. 



' Von Mohl, Botan. Zeitg. 1849, Tab. IX ; Chatin, Anatomie comparee des V^g^taux, Plantes 

 parasites, Tab. 80, 83. 



' Chatin, I.e. Tab. .<;, 6, 57, 58, 59, 64, 69, 70, 72, 77, 78, 87, 109, no. 



' Pfitzer, Pringsh. Jahrb. VII. p. 549. 



• Duval- Jouve, Bulletin de la Soc. Bot. de France, XV. (1868) p. 139. 



' Loew, de Casuarinearum caulis foliique evolutione at .^-tructura, p. 35. 



» Rudolplii (Anat. p. 91) speaks of stomata on the anthers of Lilium bulbiferum ; Unger 

 (Exanth. p. 127) on those of Capsella bursapastoris, 'in a pathological state'; on the integument 

 of Canna, Schleiden, Beitr. p. 10 ; also on the outer margin of the seed in Tulipa, Czech, Botan. 

 Zeitg. 1865, p. 10 (.—They exist on Perianths, both with and without chlorophyll, in many 



