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 
Aralia papyri/era, 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 fraxinifoUa ; 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 Verbascum 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 Palece 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 diff'erent forms, e.g. the pappus-like hairs of 
Hiera cium aurantiacum and Azalea indie a, 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 Eloeagnus, Hippuris, and Pinguicula, Tufts of hairs arise when 
the mother-cell of the hair vi/hich 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 
prominence 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 
