EPIDERMIS. 



71 



Still they are present in a considerable number of exceptional cases. Thus on 

 the foliao-e leaves of Coffea, Viburnum Avabaki, Cocculus laurifolius, Cinnamomum 

 aromaticum, Camellia japonica \ and of Grasses ^, where some of them are arranged 

 perpendicularly to the outer surface ; but on the undulating corners they are directed 

 obliquely outwards from the lumen of each cell, and facing the neighbouring cell, so 

 that those of two neighbouring cells cross. They occur also in Abies ^ Cycas*, 

 Lycopodium pinifolium\ and Equisetum hiemale (comp. Fig. 24 B^"). The walls of 

 the elongated epidermal cells of the upper side of the leaf of Acropteris australis 

 show a spiral striation, as the result of peculiar pitting (comp. Sect. 30). The free 

 surface of the outer walls is often quite smooth : but is not uncommonly uneven 

 by reason of small thickenings protruding outwards : short warts, e.g. in species of 

 Equisetum, leaves of Sparganium ramosum. Aloe verrucosa, Radula, Crassulaceee 

 (comp. Fig. 20, p. 53), &c. : bands, which are relatively broad and blunt, e.g. leaf of 

 Helleborus niger, foetidus ^ Dianthus Caryophyllus, plumarius, or thin and sharp, as 

 in very many leaves and petioles, e.g. Allium Cepa, Eucomis, Rumex Patientia '^, 

 obtusifolius. The bands often run nearly straight and parallel, and are then usually 

 lono-itudinal relatively to the whole body, rarely (Eucomis) they are transverse ; not 

 uncommonly they are wavy and branched (e.g. Helleborus, Pirus communis), and in 

 the majority of cases they are continuous from one cell to the next. 



The wall of the stomaial cells'^ is usually, but not always, thinner on the average 

 than that of the adjoining epidermal cells. It is in most, and one may say in regular 

 cases, unequally thickened in such a way that a strongly thickened ridge runs along 

 the entrance and exit of the slit (Fig. 23). These ridges protrude on the free sur- 

 face as the above described ridges of en- 

 trance and exit, which are sharp-edged and 

 concave towards the slit ; rarely both are 

 almost equally strong (Lilium candidum, 

 Ficus elastica); usually the ridge of entrance 

 is much stronger than the other, and in su- 

 perficial stomata of tough leaves it often 

 takes the form of a high and thick wall, e.g. 

 Clivia nobilis, many Proteaceae, Pholido- 

 phyllum zonatum (Fig. 12, p. 37), Epidendron 

 ciliare, Octomeria, Sarcanthus rostratus, &c. 

 The ridge of exit is often extremely small 

 (leaf of Pholidophyllum, Dianthus caryophyl- 

 lus, Lomatia longifoHa, Sparganium ramosum), or is not present at all (comp. p. 35). 

 The thickened ridges either protrude into the cavity of the cell as flattened swellings, 

 or not at all. The remainder of the wall of the guard-cell, that is the convex side 



FIG. 25. — HyacLnlhub orientalts ; leaf, transverse sec- 

 tion ; € — e epidermal ceils; ^ entrance of the stoma, 

 which is cut in median transverse section ; t the respi- 

 ratory cavity, between the parenchymatous cells /. 

 (800). From Sachs' Textbook. 



1 Kraus, I.e. p. 318. * Von Mohl, Verm. Schr. Taf. IX. 



3 Thomas, Hildebrand, I.e. * Von Mohl, /. e. Taf. X. 



5 Sanio, Linnaa, 29, p. 169. ^ Von Mohl, Verm. Schr. Taf. IX. 6-8. 



'' Von Mohl, I.e. figs. 3-5. 



8 Compare the papers quoted above, sect. 5, especially Von Mohl, Spaltoffn. d. Proteaceen ; idem, 



Botan. Zeitg. 1856, I.e.; and the large series of good representations in Strasburger's work, Pring- 

 sheim's Jahrb. V. 



