lOO MORPHOLOGY OF TISSUES. 



extraordinary 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. Among the simplest forms are also the Root-hairs which grow from 

 the epidermis of true roots or underground stems, as Pteris aquilina, Equisetaceae, &c. ; 

 they are thin-walled protuberances of the epidermal cells which lengthen by growth 

 at the apex, or only branch exceptionally, as occurs sometimes in the turnip. 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. The structure 

 is similar of the woolly hairs which appear 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 

 Jralia papyrifera, where they form a felt easily wiped off from the newly unfolded 

 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 at the apex, and are usually separated by a septum 

 from the mother-cell. When the free outer wall of unicellular hairs exhibits 

 greater apical and surface-growth at two or more spots, branched forms result 

 with a continuous cavity. The papillose bulging of an epidermal cell may become 

 separated by a septum; the hair then consists of a basal cell and a free hair- 

 cell, as in Anemia fraxinifolia ; but the separated papilla may also become seg- 

 mented 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 

 blanched structures with whorled or alternate branches, e.g. in Verhascum Thapsus 

 and 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 are the result. To this form belong, for example, the 

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

 the divisions in the young hair may be so arranged that it forms at length a tissue, 

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

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

 sanguineum. 



The papilla which projects above the epidermis and is separated by a septum often 

 becomes divided by vertical and radial walls, and expands in a disc-like manner, so 

 that the head consists of a radially arranged disc of numerous cells ; thus arise Peltate 

 Hairs, such as those of Elceagnus, Hippuris, and Pinguicula. Tufts of hairs arise when 

 the mother-cell of the hair which belongs to the epidermis divides into several cells 

 lying close to one another ; each of these then grows independently into a hair, as is 

 shown in Fig. 83, which is supplemented by Fig. 42, p. 43. 



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

 hair, and subsequently also in the epidermis ; the hair itself is then borne on a conical 

 promTnence or protuberance of the leaf or stem, into which its lower part is often 

 deeply implanted ; as, for instance, in the stinging hairs of the stinging-nettle. 

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

 hop are inserted at their base into a protuberant mass of tissue, while the upper 

 part grows into two opposite sharp points. Such double-pointed unicellular hairs 

 occur also on the under-side of the leaf of Malpighia urens ; they are from five to 

 six mm. long, fusiform, very thick-walled, and are attached to the epidermis (without 

 any protuberance) by their central part. In this case they easily become detached, 

 and remain sticking in the skin of the hand which touches the leaf. (For further details 

 on the Morphology of Hairs, see Sect. 21.) 



It is very common for hairs to be secreting organs. Such are the Stinging Hairs 

 already mentioned of Urticaceae, many Loasaceae, &c., as well as the short hairs of 

 some Urticaceae which contain cystoliths. But the most remarkable examples are 



