EPIDERMIS. 51 



doubt, since here the guard-cells die off at an early stage (e. g. Tropasolum, Colocasia, 

 Aconitum, &c.), or disappear altogether (Hippuris, Callitriche) ; other cases certainly 

 require confirmation. Finally, there is often besides this a considerable difference of 

 form and size from the air-pores, which sometimes occur on the same epidermal 

 surface with them. 



The water-pores always lie over the ends of the vascular bundles, the structure 

 of which is described in Chapter VIII ; and therefore usually near the margin of the 

 leaf, on the teeth, and, in most known cases, on their upper side : more rarely on 

 other parts of the leaf-surface, singly or in groups, in the latter case often between 

 epidermal cells, which differ from the rest in special form and (smaller) size. Also 

 in closely related species there is, according to the species, in one case a single pore, 

 in another a group of pores. The higher their number at one place, the smaller on 

 the average is their absolute size, and also the difference in size between them and 

 the air-pores connected with them. The absolute size is in extreme cases very 

 considerable, by far exceeding the maxima for the air-pores. 



According to their shape, one can distinguish two extreme forms of water- 

 pores ; on the one hand those with almost semicircular guard-cells, and with a 

 slit always quite small and skorl (Crassula, Ficus, Saxifraga) ; and on the other hand 

 those with a very large, long sht which is always found open, e. g. the huge 

 stomata on the leaves of Aroidese, Papaveraceae, and Tropaeolum. The largest 

 of the latter are not uncommonly examples of the early death of the guard-cells pre- 

 viously mentioned. 



The occurrence of water-pores is a very widespread phenomenon, to which the not 

 very lucid statement of Trinchinetti on ' Glandulse periphyllae ' refers \ Recently Met-, 

 tenius^ and after him RosanofT^, Borodin*, and Magnus^, have paid especial attention 

 to them. 



The form with relatively longer slit is known among land plants in the case of the 

 water-dropping apices of leaves of the Aroidese; in Colocasia antiquorum ^ Caladium 

 odoruin ', and C. esculentum **, there are two or three enormously large, wide, open 

 pores. 1"he water-dropping spot on the middle of the under side of the hair-like leaf- 

 apex of Richardia sthiopica has numerous widely open stomata, which are larger and 

 rounder than the air-pores. Further, of Dicotyledons, the following cases, mostly on the 

 authority of Mettenius, are to be mentioned. 



One relatively very large, wide, open pore is to be found at the apex of the leaf-teeth 

 of the Fuchsias (Fuchsia globosa, &c,j, Primula sinensis (rarely 2), (comp. below. Chap. 

 VIII); on the upper side of each tooth (and of the apex of the leaf) in Saxifraga 

 orientalis, cuscutaeformis, punctata, Heuchera, Mitella, Soldanella Clusii, Primula auri- 

 cula, marginata, acaulis, species of Aconitum and Delphinium, Eranthis ; one or two in 

 the same position in Sambucus nigra, Valeriana sambucifolia, Doronicum Pardalianches, 

 Ribes triste, Prunus Padus; three in Cyclamen; a group of 3-6 of them in the same 

 position in Ulmus campestris, Carya amara, Crataegus coccinea, Helleborus niger, 

 Geranium macrorhizum; of 6-8 in Crepis sibirica, Helenium autumnale, Verbesina 

 virginica ; an about equal, but not quite definite, number of them at the same point in 



^ Linnsea, Literatuiblatt, pp. 11, 66. ^ Filices horti Lipsiensis, pp. 0, 10. 



' Botan. Zeitg. 1869, p. S83. * Ibidem and 1S70, p. 841. 



^ Botan. Zeitg. 1871, p. 479. 



" Duchartre, Ann. Sci. Nat. 4 Ser. torn. XII. p. 264, pi. 17. 

 ' E. de la Rue, Botan. Zeitg. 1866, p. 321. * Mettenius, I.e. 



E 2 



