54 CELLULAR TISSUE. 



The depressions on the upper surface of the leaf of some species of Ficus (F. neriifolia, 

 diversifolia, Porteana, Cooperi, eriobotryoides, leucosticta, &c.) have in the main the 

 same structure as in Crassula. 



The openings which Tr^cuP describes on the large prickles on the leaf-nerves and 

 petiole of Victoria regia may be here supplementarily mentioned, being doubtful as 

 regards their structure, and requiring further investigation. These prickles enclose a 

 thin vascular bundle, which ends under their apex, and at the apex itself is to be 

 found a depression with one circular opening (ostiole). 



Finally, while referring to later chapters, it must be remarked that the excretion of 

 water or solutions of hme over the ends of vascular bundles is not always connected 

 with the presence of water-pores. 



Sect. 9. Gaps in the epidermis other than stomata and their modifications 

 occur normally only in rare exceptional cases. In connection with the water-pores 

 there may here first be mentioned the cracks, which occur regularly at the apex of 

 leaves of the grasses (seedlings of Zea, Secale, Triticum, &c.) ; from these drops of 

 water are expressed. They arise by irregular tearing of the originally cowl-like apex 

 of the leaf, when this spreads itself out flat as it unfolds. Gaps of another sort, as 

 found by Milde and King '■', occur on the middle part of the winged base of the leaf 

 of Osmunda regalis, cinnamomea, Claytoniana, Todea rivularls, and on the ligule of 

 the base of the leaf of Isoetes lacustris. The undulating lateral walls of the epidermal 

 cells leave intercellular spaces between them, which are elliptical or circular in surfacg- 

 view, and are often as large as the cells themselves. Their distribution is irregular : 

 often many are near one another, even two between two cells, often there are none 

 for a width of several cells. They pass through the entire thickness of the epidermis, 

 and open into the intercellular spaces to be found below them. They are filled either 

 with air or with a colourless jelly of unknown origin. 



No further examples can be here adduced of gaps in the epidermis, which are not to 

 be classed with stomata; mistakes formerly made with regard to Salvinia and Azolla 

 have been corrected ; the supposed round pores of the Pleurothallideae, again reproduced 

 by Unger^, have been proved to be the insertions of sunken hairs*; and Luerssen has 

 recently shown that the large pores, visible with the naked eye in the leaf of Kaulfussia, 

 are typical stomata, of huge size and wide cavity, with collapsing guard-cells, and sur- 

 rounded by 2-3 rings of subsidiary cells '. 



Sect. 10. Such outgrowths above the outer surface of the epidermis as do not 

 belong to the cell wall alone are termed, in the plants with which we are concerned, 

 Hair-strudures {Trichomes, appendages of the Epidermis). These spring from cells of 

 the epidermis, and are derived from them. 



We may distinguish as typical forms of hair-structures, Bladders (Papulae), 

 Hairs (Pili, Setae), Scales (Squamae, Lepides, and Paleae), and Shaggy hairs (Villi), 

 Warls, and Prickles. These forms are characterised by simple relations of shape, 

 which mostly explain themselves according to their meaning borrowed from the 

 language of ordinary life, and by equally simple differences of structure : Bladders 



' Ann. Sci. Nat. 4 Ser. I. 156, p. 13, fig. lo. 

 " Milde, Monogr. generis Osmundoe, p. 86. 

 ' Anat. und Physiol, p. 194. 



' Meyen, in Wiegmana's Archiv, 1837, 1, p. 419; Schleiden, ibid. 1838, 1; Beitr p 5 

 ' De Vriese et Harting, Monogr. des Marattiacees, p. 14, Taf. V. D.-Lnerssen, Botan. Zeitg. 

 1873, No. 40. ^ 



