50 CELLULAR TISSUE. 



Sagittaria sagittsefolia has stomata on both sides of the aerial leaves, 4-5 times as 

 many below as above, on an equal area of surface ^ : on the floating leaves they are very 

 rare on the under surface, but numerous above. — In the terrestrial forms of the Eucalli- 

 triches the stem and both leaf-surfaces are rich in stomata^, on the submerged forms they 

 are absent on the stem, and occur only solitary on the leaves, on the floating leaves they 

 are numerous on the upper side. A similar relation to that in Sagittaria occurs in the 

 aerial, and the casually or abnormally produced floating leaves of Ranunculus sceleratus. 

 An old statement of De Candolle, according to which leaves of Mentha developed 

 under water have no stomata, is doubtful, and decidedly opposed by Rudolphi '. These 

 facts are in accordance with the constant absence of stomata on certain submerged 

 species, and their presence on closely-related, terrestrial species, e.g. in the genus 

 Isoetes. 



How far the finer gradations of distribution are directly caused by the mode of life 

 and condition of vegetation requires careful investigation ; in which, besides experi- 

 mental treatment, it is important to compare, not, as has hitherto usually been the case, 

 a large number of casually selected plants, but such as are closely related. By the latter 

 method Pfitzer* has obtained the following result for a large number of indigenous 

 grasses : that for these plants the number and distribution of the air-pores, together 

 with the form of the surface and internal structure of the whole leaf, stand pretty 

 generally in definite relation to the wetness of the locality. On both flat leaf-surfaces are 

 numerous stomata in all marsh- and water-grasses observed (9 species, e. g. Phragmites 

 communis, Alopecurus geniculatus) : in numerous meadow- and weed-grasses (34 species, 

 e. g. Alopecurus pratensis, Anthoxanthum odoratum, Hordeum murinum, Triticum repens): 

 among the latter Festuca elatior is a remarkable exception, in that stomata appear only on 

 the upper side of the leaf. Almost all grasses inhabiting very dry localities have leaves 

 with well-marked longitudinal folds; the surface is therefore marked with long and narrow 

 furrows, and the stomata are almost exclusively on the sides of the grooves of the upper 

 side of the leaf (12 species, e.g. Aira caryophyllea, flexuosa, Elymus arenarius, Stipa pen- 

 nata). Koeleria cristata and Agrostis vulgaris have, with leaf structure otherwise re- 

 sembling the latter category, numerous air-pores also on the under side of the leaf. 

 The remaining 14 investigated species inhabit bright glades, sunny hills and grass plots; 

 they have leaves flat on both sides, and, like the above meadow-grasses, some of them 

 have stomata on both sides (Avena pratensis, Holcus mollis, Phleum Boehmeri, Poa 

 bulbosa, compressa, nempralis, Milium effusum), others — perhaps, with exception of 

 Triodia, plants which live only in shady situations — have them only on the upper 

 side (Brachypodium silvaticum, Festuca gigantea, heterophylla, Melica nutans, uniflora, 

 Triodia decumbens, Triticum caninum). Milium is an exception as compared with 

 these. 



Sect. 8. Numerous phanerogamic plants, of the most various adaptation, have 

 usually besides the air-pores other stomata different from these, which may be 

 called Water-stomata or -pores ", since, under definite normal conditions, they serve as 

 points of exit for excreted drops of water. These drops in many cases hold in 

 solution large quantities of calcium carbonate, which dries into small scales. These 

 differ accordingly from the air-pores by the slit (and the respiratory cavity below 

 it) being, at least at times, filled with water. They are further characterised, ^s far 

 as investigations extend, by their guard-cells being immovable, that is, they are 

 incapable of independent intermittent widening. In many cases this is beyond 



' Karelstschikoff, I.e. 2 Hegelmaier, I.e. 



' Anat. d. Pfl. p. 69. » /.f. Pringsheim's Jahrb. VII. 



^ [Cf. Langer, Botan. Zeitg. 1879, p. 5 1 1 .—Gardiner, Quart. Journ Micr. Sci. 1881, p. 407.] 



