EPIDERMIS. 49 



Abies pectinata, Neriurn, Rhododendron, Ilex, Ficus, Begonias, and many others, have 

 stomata as a rule exclusively on the under surface : the same is the case with many firm 

 herbaceous leaves, as in Glechoma hederacea, Asperula odorata, TroUius europaeus, &c. 

 (Karelstschikoff), Betula alba, Pirus communis, Carpinus, &c. (Morren). Rarely the 

 relation is reversed, and with it the inner structure of the leaf also : there are herba- 

 ceous, and even leathery leaves (Pinus sylvestris and its allies, Eryngium maritimum L., 

 &c.) with more stomata on the upper than the under surface ; or with the upper 

 surface exclusively bearing stomata, the under surface without them, as Pinus strobus, 

 Thuja spec, Passerina hirsuta', filiformis, ericoides, and many Graminese with a deeply- 

 grooved, upper side of the leaf, e.g. Aira flexuosa, Calamagrostis epigeios, Stipa pennata, 

 &c., which will be mentioned below. Flat leaves which stand vertically, and most 

 fleshy juicy ones (Crassulacese, many Monocotyledons), bear stomata as a rule, though not 

 without exception, on both sides ; either they are equally numerous on both sides, or they 

 preponderate on one side or the other. In this they answer to their centric structure. 

 Leaves, which float on the surface of the water, have stomata exclusively on the upper 

 side, or at least chiefly so, as Callitriche (Hegelmaier /. c), the floating leaves of Sagit- 

 taria'. Ranunculus sceleratus '- Further general rules, or laws for their distribution and 

 their relations as to number, cannot for the present be laid down. No general decisive 

 differences, either according to natural affinity and habit, or other conditions of structure 

 of the epidermis, hold good throughout. Further, the proposition that, the more sto- 

 mata there are on a surface, the less their size, and -vice -versd, is not without exceptions. 

 Of the observed cases of variation and conformity many cannot be referred directly 

 to immediate adaptation. For instance, of the two Lathraeas, above cited, which are of 

 the same habit, with similar members and structure, the one has many stomata on stem 

 and leaves, the other none. 



But on the other hand, the occurrence and distribution of air-pores yields many re- 

 markable examples of the change of structure by direct, often individual adaptation. 

 This is especially the case for the amphibious water plants, and indeed all these, though 

 they belong to the most different families and genera, as Marsilia, Sagittaria, Polygonum, 

 Callitriche, Myriophyllum, Hottonia, Nasturtium, Ranunculus, show the same beha- 

 viour, viz. that where numerous stomata are found on surfaces developed in the air, 

 corresponding surfaces developed under water have fewer stomata or none at all. 



Marsilia quadrifoliata and other species of the genus * have, according as their habitat 

 is submerged or not, floating leaves, with their upper side only exposed to the air and 

 borne by thin delicate stalks, or leaves, borne on short stout stalks, rising into the air. 

 These aerial leaves have on both surfaces almost equally numerous stomata, sunk slightly 

 beneath the outer surface, between the strongly sinuous epidermal cells. In the floating 

 leaves only the upper surface bears stomata, and on the same area of surface more than 

 double as many as the aerial leaf. They lie between less sinuous epidermal cells (comp. 

 above, p. 31), and in M. quadrifoliata, pubescens, diffusa, Ernesti, not depressed ; in other 

 species, as M. Drummondii, macra, they are depressed like those of the aerial leaves. 



A similar difference exists between the aerial and floating leaves of Polygonum 

 amphibium, and Nasturtium amphibium *. The petiole and laciniae of the cut leaves of 

 Ranunculus aquatilis, divaricatus, Myriophyllum, and Hottonia ', which in their normal 

 submerged state are without stomata, form numerous stomata when they develop in the 

 air ^Land form). 



' Caruel, Nuovo giomale botan. Italianc, I. p. 194. 



' Hildebrand, Botan. Zeitg. 1870. 



' Ascherson, Botan. Zeitg. 1873, pp. 422, 631. 



' Hildebrand, Botan. Zeitg. 1870, p. [, Taf. i.— A. Braun, Monatsber. d. Berlin. Acad. 1870, 

 p. 670. — On the inconstant or exceptional behaviour of M. j^gyptiaca and some others, compare 

 the same. 



« Hildebrand, /. c. ; Karelstschikoff, /. c. ° Askenasy,. I. c. 



