EPIDERMIS. 57 



the youngest leaf ^ On the same surface, their formation begins at an earlier stage 

 of development than that of the stomata. ' The succession of appearance of the hair- 

 structures follows the development of the part of the plant which bears them, but not 

 so thoroughly that the hairs, in their successive appearance, arrange themselves 

 exactly according to the direction of the advancing growth of the leaf which bears 

 them. Not uncommonly new hairs grow out between those already formed.' Most 

 hairs on the parts named attain their full development with or before the complete 

 unfolding of the bud. The thick covering of hairs, scales, and shaggy hairs in the 

 bud-condition, is generally known. As the bud unfolds, the thickness of the covering 

 decreases, partly as a result of the separation of the persistent hairs on the growing 

 surface ; but partly through the disorganisation of hairs present in the bud during the 

 unfolding, so as to leave behind only rudiments on the unfolded parts, or hardly that^- 

 Even parts, which after unfolding are completely bare, may be hairy in the bud, e. g. 

 the leaves of Ficus elastica^ (Comp. Fig. i%A, p. 44.) 



We may accordingly distinguish between evanescent, transitory hairs which are 

 peculiar to the bud, and persistent hairs. Among the latter we may again distinguish, 

 as will be shown below (Sect. 13), between such as persist as living hairs, and 

 others which are adherent but dry. 



In roots the case is different from that described. It is a universal rule, 

 that here the hairs always appear on that part which is just ceasing to unfold, 

 i. e. to extend. 



The above sentences will give the general points of view for the anatomical con- 

 sideration of the differentiation of the hair-structures. Under this head are ranged 

 structures rich in peculiarities, which have been the object of many works, and therefore 

 have a huge literature to show. In older times more especially the forms, articulation, 

 and functions of the hairs, which do not here concern us, were dealt with*; in more 

 recent, and the latest times, investigations on the history of development are considered 

 of more importance ^ I cite below for the time up to 1867 only the chief works, and 



' On this fact, which need not be further noticed here, compare Hofmeister, Die Lehre von der 

 Pflanzenzelle, pp. 411, 545, and Rauter, Entwickl. einig. Trichomgebilde, p. 33. [Further, cf. Von, 

 Hbhnel, Botan. Zeitg. 1882, p. 145 ] 



" Compare Hanstein, Botan. Zeitg. 1S67, p. 697 ff. 



° Schacht, Abhandl. d. Senckenbergischen Gesellsch. I. 



* Guettard, Memoires sur les glandes des plantes, &c. Eleven treatises in the Memoires de 

 I'Acad. Royale des Sciences, Paris, 1 745-1 759; altogether 560 quarto pages. Compare A. Weiss, 

 /. c. — F. V. P. Schrank, Von den Nebengefassen d. Piianzen, Halle, 1 794, 8vo., with 3 plates. — Rudolphi, 

 Anatomie, p. 117 ff. — P. de CandoUe, Organographie vegetale, I. p. 108. — B. Eble, Die Lehre von 

 den Haaren in der gesammten organ. Natur. Bd. I, Wien, 1831 (only known to me from references). 

 — Meyen, Secretionsorgane d. Pflanzen, Berl. 1837. — Physiologic, Bd. I and II (1838-1839).— Bahrdt, 

 De pilis plantarum. Diss, inaug.; Bonn, 1849. — A.Weiss, Die Pflanzenhaare (Abdr. aus Karsten's 

 Botan. Untersuchungen, Bd. I); 306 pages, 13 plates, 8vo. 



^ Hanstein, Ueber die Organe der Harz- und Schleimabsondening in den Laubknospen. Botan. 

 Zeitg. 1868. — J. Rauter, Zur Entwickelungsgeschichte einiger Trichomgebilde; Wien, 1871, with 

 9 plates (from Denkschr. d. Wiener Acad. Bd. XXXI). — J. Martinet, Organes de secretion des vege- 

 taux; Ann. Sci. Nat. 5 serie, torn. 14 (1872), pp. 91-232, pi. 8-21. — O. Uhlworm, Beitr. z. Entw. 

 der Trichome, Botan. Zeitg. 1873. — Further, N. Kauffmann, Ueber die Natur der Stacheln ; Bullet. 

 Sci. Nat. de Moscou, tom. XXXII. p. 301 (1859) ; Warming, Sur la diiference entre les trichoiiies 

 et les epiblast^mes d'un ordre plus eleve (Abdr. aus Kopenhagen. Videnskab. Meddelelser), Copen- 

 hague, 1873. — C. Delbrouck, Ueber Stacheln und Dornen. Diss., Bonn, 1873. — S. Suckow, Ueber 

 Pflanzenstacheln, etc. Diss., Breslau, 1873. [Further, Reinke, Anatomie d. an Laubblattern vor- 

 kommenden Secretionsorgane, Pringsheim's Jahrb. X. p. 119.] 



