THE EPIDERMAL TISSUE. 



85 



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

 traordinary variation. The first indication of the formation of hairs occurs in the 

 papillose protuberances of the epidermis of many petals, to which their velvety ap- 

 pearance is due. To the simplest forms belong also the root-hairs which grow from the 

 epidermis of true roots or underground stems {Pteris aquilhia, Equisetum, &c.) ; they are 

 thin-walled bag-like protuberances of the epidermis-cells which lengthen by growth at 

 the apex, or only branch exceptionally (as sometimes in Brassica Naptis). In Vascular 

 Cryptogams their wall readily acquires a brown-red colour; their length of life is 

 usually short, and when they die all trace of them disappears. In a similar manner 

 the woolly hairs behave which appear early on the leaves and internodes of vascular 

 plants, while still in the bud, especially Dicotyledons. On the unfolding of these 

 organs they commonly fall off and disappear, as in the horse-chestnut, Rhododendron, 

 and Aral'm papyri/era, where they form a felt easily wiped off from the freshly developed 

 leaves ; in other cases they remain as a woolly coating, especially on the under-sides of 

 leaves. In prickles the wall is mostly thicker, silicified, and hard ; they are shorter 

 than the woolly hairs, pointed upwards, and a septum separates the prominence from 

 the mother-cell. When two or more points endowed with a greater power of growth 

 in their surface and apex arise on the free outer wall of unicellular hairs, branched 

 forms result with continuous cavity. The papillose bulging of the epidermis-cells 

 may become separated by a septum ; the hair then consists of a basal cell fixed in the 

 epidermis and of a free hair-cell (as in Aneimia fraxinifolid) ; but the separated papilla 

 may also become segmented by the formation of more or less numerous septa, when 

 the hair grows considerably in length, and thus arise segmented hairs (as e.g. on 

 the filaments of Tradescantia). Sometimes the segments form lateral shoots ; and 

 thus arise tree-like branched structures with whorled or alternate branches {e.g. Ver- 

 bascum Tbapsits, Nicandra physaloides). If longitudinal divisions occur in the segment- 

 cells of the hair, or if the hair continues to grow by an apical cell which forms segments 

 on two sides, flatly expanded hairs arc the result. To this form belong, for example, the 

 so-called palcae of Ferns which sometimes entirely cover the younger leaves. Finally 

 the divisions in the young hair may be so arranged that it presents at length a 

 tissue, which on its part may again assume different forms, e.g. the pappus-like hairs 

 of Eieracium aurantiacum and Azalea indica, the capitate hairs of Korrea and Ribes 

 ianguineum. 



Very commonly the terminal cell of a segmented or the end of a solid hair (j. e. of 

 one consisting of a mass of tissue), swells in a globular manner, and then usually forms a 

 multicellular gland, while the cells of the head produce peculiar secretions. (On these 

 Glandular Hairs cf. sect. 17, (b),) Not unfrequently the papilla which projects above the 

 epidermis and is separated by a septum becomes divided^by vertical and radial walls, 

 expanding in a disc-like manner, so that the head consists of a radially arranged disc 

 of numerous cells; thus arise the peltate hairs, e.g. of Eleagnus, Hippuris, and Pin- 

 guicula. Tufts of hairs arise when the mother-cell of the hair which belongs to the 

 epidermis breaks up early into several cells lying close to one another ; each of which 

 then grows independently into a hair, as is shown in Fig. 72, which is completed by 

 Fig. 44, p. 43. 



Not unfrequently a luxuriant growth of the parenchyma takes place beneath the 

 hair ; and this is imitated also by the epidermis ; the hair itself is then borne on a peg- 

 shaped prominence or protuberance of the leaf or stem, and is often deeply implanted 

 into it in its lower part ; as, for instance, in the prickles (stinging hairs) of the stinging- 

 nettle. Thus also the prickles (climbing hairs) on the six projecting angles of the 

 stem of the hop grow into a large basal protuberant mass of tissue, while the hair-cell 

 grows in opposite directions into two sharp points. Such double-pointed unicellular 

 hairs occur also on the under-side of the leaf of Malpighia urens ; they are 5-6 mm. long, 

 fusiform, very thick-walled, and grow into the epidermis by their central part (without 

 protuberance). In this case they easily become detached, and remain sticking in the 



