84 MORPHOLOGY OF TISSUES. 



potash. In submerged organs and roots it is very thin, difficult to be seen immediately, 

 but 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 wall of the epidermis-cells lying 

 beneath the cuticle is strongly often enormously thickened ; while the inner-walls re- 

 main thin, the lateral walls are usually strongly thickened outwardly, becoming inwardly 

 suddenly thinned. The thick portions of the wall are usually differentiated into at 

 least two shells ; — an inner thin shell, immediately surrounding the cell-cavity, shows 

 the reactions of pure cellulose, while the epidermal layers 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 layers of cuticle extend downwards in the thick part of the 

 side-walls, in which case the middle lam*ella sometimes behaves like the true cuticle, 

 with which it is in contact on the outside. Like the isolated cells of the cuticle (pollen- 

 grains, spores), the epidermis has also a tendency to form projecting lumps, knots, 

 ridges, &c., but they almost always remain very insignificant-, and are best seen on a 

 superficial view ; as, for example, in many delicate petals (cf. sect. 4, (e)). 



According to the recent researches of De Bary, particles of wax are deposited in the 

 substance of the cuticular layers of the epidermis which cannot be seen on section, 

 but separate in the form of drops when warmed to about 100° C. This deposit of 

 wax (often combined with resin) is one of the causes 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, forming the so-called bloom on fruits and some leaves, or as a continuous shining 

 coating, which is reformed 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) of quantities of delicate 

 minute rods or needles, e. g. the white-dusted Eucalypti, Acaciae, many Grasses, &c. ; 

 or of granules collected into several layers, as in Kkinia jicoides and Ricinus communis ; 

 these are aggregated wax-coatings. (2) Simple granular coatings consist of grains iso- 

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

 Iris pallida, Allium Cepa, Brassica oleracea, &c. (3) Coatings of minute rods consisting 

 of thin, long, rod-shaped particles, bent above or even curl-shaped, and standing perpen- 

 dicularly upon the cuticle, e. g. Eeliconiafarinosa and other Musaceae, Cannaceae, Saccha- 

 rum, Beyiincasa cerifera, leaves of Cotyledon orbicularis. (4) Membrane-like layers of wax 

 or incrustations ; (a) as a gritty glazing in Sempervivum, Euphorbia Caput-Medusce, T:huja 

 occidentalis ; (b) as thin scales, in Cereus alatus, Opuntia, Portulaca oleracea, Taxus 

 baccata ; (c) as thick connected 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 incrus- 

 tations attain a thickness of 5 mm. ; those on the stem of Chamcedorea Schiedeana are 

 thinner, but of similar structure. According to Wiesner (Bot. Zeitg. p. 771, 1871), 

 these flakes of wax consist of doubly refractive four-sided prisms standing perpendi- 

 cularly close to one another. 



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

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



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

 Laborat. by ICarsten, 1867.— J. Hanstein, Bot. Zeitg. p. 697 et seq., 1868.— Rauter, Zur Entwickel- 

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

 des vegetaiix. Ann. des Sci. Nat. Fifth series, vol. XIV, 1871.] 



