J40 



EXTERNAL CONFORMATION OF PLANTS. 



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 thin and long, destitute of chlorophyll, 

 and otherwise dissimilar to the thallus which produces them. On the other hand struc- 

 tures 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 epidermis cells, but consist of massive 

 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 distin- 

 guished 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 {e.g. of Euphorbiaceae, Ricinus). They resemble the leaves and branches of 

 Phanerogams in their origin, but hairs in their later formation, and in their occurrence 

 on stems as well as on leaves, and in their irregular disposition. For spines, which 

 must not be confounded with prickles, cf. sect. 28. 



Sect. 23. The term Root^ is applied in botanical morphology, in contrast 

 to its use in popular language, only to such outgrowths of the substance of the 



Fig. no. — Longitudinal section of the young 

 primary root of the embryo of Marsilea salvatrtx; 

 TVS the apical cell, 'uh', Tvh" , Ti'h'" the still simple 

 root-cap ; x,y the last segments of the substance 

 of the root ; z t intercellular spaces. 



Fig. III.— Longitudinal section of a somewhat older prhnarj- root of MarsUea salvatrix; ws apical cell; whl + Tvhl the first, 

 whZ-ir-whi the second, wh^ the third layer of the root-cap ; each layer now consists of two divisions ; xy the youngest segments 

 of the substance of the root ; o epidermis ; £^J' fibro-vascular bundles ; h the part of the root-cap which extends furthest back. 



plant as are clothed at their growing apex with a layer of tissue, the Root-cap 

 already described in sect. 19. Roots do not form leaves or other exogenous 



^ Niigeli und Leitgeb in Nageli's Beitriigen zur wissen. Bot. Heft IV, 1867. — Hofmeister, 

 Morphologic der Gewebe, sect. 5. Leipzig 1868.— Hanstein, Botan. Abhandlungen, Heft I. Bonn 

 1870. — Dodel, Jahib. fiir wiss. Bot. VII, pp. 149 et seq. — Reinke, Wachsthnmgeschichte der Phanero- 



gamenwurzel in Hanstein's Botan. Untersuchungen, Heft HI. 

 Tieghem : Ann. des Sci. Nat., 5th series, XIII, 1S71.] 



Bonn 1871. — [La Racine: Ph. van 



