THE EPIDERMAL TISSUE. 



99 



rendered visible by iodine and sulphuric acid. The trua cuticle is much thicker in 

 aerial stems and leaves; it may be obtained in them even in large lamellae by decay 

 or solution of the subjacent cells in concentrated sulphuric acid. In many cases, and 

 especially in stout leaves and internodes, the outer cell-wall layers of the epidermal cells 

 which lie beneath the cuticle are strongly, often enormously, thickened ; while the inner 

 Malls remain thin, the lateral walls are usually strongly thickened towards the surface, 

 becoming suddenly thinner towards the inside. The thick portions of the wall are 

 usually differentiated into at least two shells; an innermost thin shell, immediately 

 surrounding the cell-cavity, shows the reactions of pure cellulose, while the layers 

 of the cell-wall lying between it and the cuticle are more or less cuticularised, and 

 the more so the nearer they lie to the cuticle. Not unfrequently these cuticularised 

 layers extend downwards into the thick part of the lateral walls, in which case the 

 middle lamella sometimes behaves like the true cuticle, with which it is in contact 

 on the outside^ Like the cuticle of isolated cells (pollen-grains, spores), that of the 

 epidermis has also a tendency to form projections, ridges, &c., but they almost always 

 remain very insignificant, and are best seen on a superficial view; as, for example, in 

 many dehcate petals (see Sect. 4 (e), p. 33). 



According to the recent researches of De Bary, particles of Wax, which cannot be 



seen on section, but which exude in the form of drops when warmed to about 100° C, 



are deposited in the substance of the cuticular layers of the epidermis. This deposit of 



wax (often associated with resin) is one of the contrivances which protect the aerial parts 



of plants from becoming moistened with water. But very frequently the wax extends in 



an unexplained manner over the cuticle, and becomes deposited there in different forms, 



:onstituting the so-called ' bloom ' on fruits and some leaves, or as a continuous shining 



fcoating, which is again formed on young organs after being wiped off, and in ripe fruits 



[of Benincasa cerifera (the wax-cucumber) appears again long after maturity. De Bary 



listinguishes four principal forms of this wax-coating. The bloom or gloss which is 



iasily wiped off consists of small particles of two forms: — (i) Quantities of delicate 



linute rods or needles, as in the white-dusted Eucalypti, Acacige, many Grasses, &c. ; 



)r of granules collected into several layers, as in Kle'mia Jicoides and Ricinus communis ; 



[these are aggregated wax-coatings. (2) Simple granular coatings consisting of grains 



[isolated or touching one another in one layer; this is the most common form, e.g. in 



Iris pallida, the onion and cabbage, &c. (3) Coatings of minute rods consisting 



lof long, slender, rod-shaped particles, bent above or even curl-shaped, and standing 



[vertically upon the cuticle, e.g. Heliconia farinosa and other Musaceae, Gannaceae, 



ISaccharum, Benincasa cerifera, leaves of Cotyledon orbicularis. (^) Membrane-like layers 



'of wax or incrustations ; (a) as a gritty glazing in various species of Sempervi'vum, 



Euphorbia Caput-Medusce,lhuja occidentalis ; (b) as thin scales, in Cereus alatus, Opuntia, 



Portulaca oleracea, the yew ; (c) as thick continuous incrustations of wax, which 



sometimes permit a finer internal structure to be recognised, similar to the striation 



and stratification of the cell-wall: Euphorbia canariensis, fruits of species of Myrica, 



stems of Panicum turgidum. On the stem of the Peruvian wax-palms, especially of 



Ceroxylon andicola, these incrustations attain a thickness of 5 mm. ; those on the stem 



of Chamcrdorea Schiedeana are thinner, but of similar structure. According to Wiesner 



(Bot. Zeitg. 1 87 1, p. 771), these flakes of wax consist of doubly refractive four-sided 



prisms standing perpendicularly close to one another. 



Hairs ^ are products of the epidermis; they originate from the growth of single 

 epidermal cells, and are present in most plants in large numbers ; when they are 

 wanting in any part of a plant, it is termed glabrous. Their form is subject to 



^ A. Weiss, Die Pflanzenhaare, in vols. IV and V of the Bot. Untersuchungen aus dem phys. 

 Laborat. by Karsten, 1867.— J. Hanstein, Bot. Zeitg. 1868, p. 697 e/ sty —Ranter, Zur Entwickel- 

 ungsgeschichte einiger Trichomgebilde. Wien 1871. [See also J. B. Martinet: Organes de secretion 

 des vegetaux, Ann. des Sci. Nat., 5th series, vol. XIV, 1871.] 



H 2 



