HAIRS (TRICHOMES). ^ OQ 



which always remain the outermost in roots, stems, and leaves, whether these 

 outgrowths occur as simple utricular protuberances, row's of cells, plates of 

 cells, or masses of tissue, or have the physiological character of woolly envelopes 

 of the young leaves, root-like absorbing organs (Mosses), glands, prickles, or 

 spore-capsules (Ferns). 



The hairs may originate from the primary meristem of the pimctum vegetationis, 

 or from young leaves and lateral shoots, if an external layer of cells has already been 

 differentiated as dermatogen, as in Phanerogams. But they may originate also in 

 much older parts the tissue-systems of which have already become further differ- 

 entiated, and which exhibit intercalary growth, because in such cases the epidermis 

 long remains generative ; e. g. produces stomata and allows of cell-division. 



When hairs spring from the piuictiun vegctatioiiis, they are usually formed after 

 the leaves, /. e. further from the apex than the youngest leaves ; but it also happens 

 in Phanerogams that they are developed above the youngest leaves and nearer to 

 the apex, the outermost layer of cells of the pwictum vegetationis having in this case 

 already become differentiated as dermatogen (as in Utricularia according to Prino-s- 

 heim). In ^Mosses and Vascular Cryptogams also, where the leaves become visible 

 long before the differentiation of the external layers of tissue, the hairs do not show 

 themselves on the surface of the stem till a later period and further from the apex. 



If the hairs arise in the neighbourhood of the apex of a punctum vegetationis 

 or on a zone of interstitial basal growth (as in the sporangia of Hymenophyllacese), 

 they may be arranged according to a definite law, which is not the case with hairs 

 that spring from older organs, or at least not evidently so. 



Hairs are always strikingly different in their form from the leaves and lateral 

 shoots of the same plant, although they sometimes bear a certain resemblance to 

 these organs of other plants. The development in size of a single hair is usually 

 extremely small compared to that of the member which produces it ; even the mass 

 of all the hairs of a leaf, a root, or a stem, is generally quite inconsiderable com- 

 pared to its weight. 



(a) The woolly and glandular hairs in 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 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. 

 The root-hairs are formed at a considerable distance from the punctum 'vegetationis 

 of the root, often 1-2 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 vas- 

 cular plants. These root-hairs of Mosses are remarkably endowed with the generative 

 principle, and behave in many respects like the Protonema, a means of propagation 

 peculiar to Mosses; like it, they produce gemmae, w^hich, w-hen exposed to light, grow 

 into leafy stems. If the root-hairs themselves come to the surface {e.g.h^ turning 

 up a sod) they put out rows of cells rich in 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 



