HAIR (tRICHOME). i6i 



(a) The woolly and glandular hairs on buds are distinguished by a remarkably 

 ipid growth ; they are often perfectly formed long before the parts of the bud unfold, 



mt then they generally die off; 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, 

 'he 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 

 Iweeks, so that the older parts of the roots of even annual plants are destitute of living 

 lairs. The existence of these hairs is connected with the activity of the roots in the 

 "ound. 

 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 

 ito rows by oblique septa, and, viewed physiologically, replace the root-system of 

 vascular plants. These root-hairs of Muscineae are remarkably endowed with generative 

 )ower, 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 out 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 

 land 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 

 pn the same sense as those of the higher plants. Nevertheless it is customary to speak in 



5uch cases also of hairs, when the outgrowths are long and slender, destitute of chloro- 

 Fphyll, 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 

 )y 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 

 •uits (according to Warming, for example, on the fruit of Datura Stramonium, and, 

 iccording to my own observations, on that of Ricinus). To the same category belong 

 Pthe ' beards ' of many petals (according to Warming, e. g. those of Menyanthes trifoliata) ; 

 the 'tentacles' on the leaves of Drosera, the sharp hairs beneath the calyx oi ^grimcnia 

 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- 

 jular distribution both as respects one another and the organ on which they grow. The 

 classification adopted by Warming (A c. p. 27), i>/2. including emergences under the term 

 trichome, and dividing this class of structures into two sub-classes, hairs and emergences, 

 iems to me, if not false, at all events inconvenient ; because it becomes impossible to 

 five 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 

 ixes, 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 phyliome and caulome, or 



^ On spines, which must not be confounded with prickles, see Sect. 28. 



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