HAIR (TRICHOME), 
i6i 
(a) The woolly and glandular hairs on buds are distinguished by a remarkably 
rapid growth ; they are often perfectly formed long before the parts of the bud unfold, 
but then they generally die olf; the persistent hairs which remain during the life of 
the leaves are formed much more slowly, and are marked by a great variety of form. 
The root-hairs are formed at a considerable distance from the growing point of the 
root, often from i to 2 cm. from the apex, and mostly die off after a few days or 
weeks, so that the older parts of the roots of even annual plants are destitute of living 
hairs. The existence of these hairs is connected with the activity of the roots in the 
ground. 
The root-hairs which spring from the stems of Mosses are marked by a very long 
continued apical growth, and often by repeated branching. They consist of cells divided 
into rows by oblique septa, and, viewed physiologically, replace the root-system of 
vascular plants. These root-hairs of Muscinese are remarkably endowed with generative 
power, and behave in many respects like the Protonema, a means of propagation pecu- 
liar to Muscineae; like it, they produce gemmae, which, when exposed to light, grow 
into leafy stems. If the root- hairs themselves are exposed to the air {e.g. by turning 
up a sod) they put oat rows of cells containing chlorophyll, on which also gemmae 
are produced. 
(b) Thallophytes, when they consist of a mass of tissue, also form true hairs, like 
Cormophytes ; but when the thallome consists only of one layer of cells, or, like Caulerpa 
and others, is unicellular, one can no longer speak of an external layer corresponding to 
the epidermis ; and its hair-like outgrowths cannot therefore be considered as trichomes 
in the same sense as those of the higher plants. Nevertheless it is customary to speak in 
such cases also of hairs, when the outgrowths are long and slender, destitute of chloro- 
phyll, and otherwise dissimilar to the thallus which produces them. On the other hand 
structures occur in highly organised plants which are closely analogous to many forms of 
hairs in their physiological, and partly also in their morphological properties, but which 
differ from true hairs in not originating from single epidermal cells, but consist of 
outgrowths of the tissue which lies beneath the epidermis, remaining however covered 
by a continuation of it. Examples of such structures, which may perhaps be dis- 
tinguished by the term Emergences, are afforded, according to Rauter, by the prickles^ 
and glandular hairs of roses, and perhaps also of the various species of Rubus. Closely 
related to these are probably the warts, tubercles, and knobs on the surface of many 
fruits (according to Warming, for example, on the fruit of Datura Stramonium, and, 
according to my own observations, on that of Ricinus). To the same category belong 
the ' beards ' of many petals (according to Warming, e. g. those of Menyanthes trifoUata) ; 
the 'tentacles' on the leaves of Drosera, the sharp hairs beneath the calyx of Agrimonia 
Eupatorium, the pappus of Compositae, &c. Larger emergences of this nature may even 
be penetrated by branches of the vascular bundles from the organs which produce them, 
as in Drosera, Datura, &c. They resemble the leaves and branches of Phanerogams in 
their origin and mode of formation, while they agree with hairs in the late period at 
which they are produced, their occurrence on stems and leaves, and their frequently irre- 
gular distribution both as respects one another and the organ on which they grow. The 
classification adopted by Warming (/. c. p. 27), 'vix. including emergences under the term 
trichome, and dividing this class of structures into two sub-classes, hairs and emergences, 
seems to me, if not false, at all events inconvenient ; because it becomes impossible to 
give any exact definition to the term trichome. The fact that emergences constitute a 
transition between trichomes, in the stricter sense of the term, and leaves or secondary 
axes, does not justify including them under the former term ; they might as well be 
treated as branches of leaves or of stems. If the occurrence of transitional structures 
were held to prevent our distinguishing certain groups of members sharply from one 
another, then the distinction must be abandoned between phyllome and caulome, or 
^ On spines, which must not be confounded with prickles, see Sect. 28. 
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