THE EPIDERMAL TISSUE. 
99 
rendered visible by iodine and sulphuric acid. The true 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 
walls 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 delicate 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, 
constituting the so-called ' bloom ' on fruits and some leaves, or as a continuous shining 
coating, 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 
distinguishes four principal forms of this wax-coating. The bloom or gloss which is 
easily wiped off consists of small particles of two forms: — (i) Quantities of delicate 
minute rods or needles, as in the white-dusted Eucalypti, Acaciae, many Grasses, &c. ; 
or of granules collected into several layers, as in Kleinia ficoides 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 
of long, slender, rod-shaped particles, bent above or even curl-shaped, and standing 
vertically upon the cuticle, e.g. Helicoriia farinosa and other Musaceae, Cannaceae, 
Saccharum, Benincasa cerifera, leaves of Cotyledon orbicularis. (4) Membrane-like layers 
of wax or incrustations ; (a) as a gritty glazing in various species of Sempervi'vum, 
Euphorbia Caput-Medusce, T.huja 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 Chamfpdorea Schiedeana are thinner, but of similar sti ucture. 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 et seq. — Rauter, Zur Entwickel- 
ungsgeschichte einiger Trichomgebilde. Wien 1871, [See also J. B. Marlinet: Organes de secretion 
des vegetaux, Ann. des Sei. Nat., 5th series, vol. XIV, 1871.] 
H 2 
