J 62, 



MORPHOLOGY OF MEMBERS. 



between caulome and root, since transitional structures occur also in these cases. The 

 occurrence of these transitional structures in nature is in itself a reason for framing exact 

 definitions. Definitions are not in themselves objects; but are means for arranging 

 objects, and for enabling us to understand them. 



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 

 plant as are clothed at their growing apex with the Root-cap already described in 

 Sect. 19. Roots do not form leaves or other exogenous foliar structures; their 

 epidermal cells, on the contrary, generally develope into long tubular appendages, 

 the Root-hairs, The apex of every root which is just beginning to be formed lies 

 beneath the surface of the organ from which it proceeds^; it is then usually 



Fig. 120. — Longitudinal section through the 

 young primary root of the embryo of Marsilea 

 salvatrix; lus the apical cell, wh', wh", ivh"' 

 the still simple layers of the root-cap ; x, y the 

 last segments of the substance of the root ; i i 

 intercellular s aces. 



Fig. 121. — Longitudinal section through a somewhat older primary root o( Marsi/ea salvatrix ; wj the apical cell ; wklx 

 •wh% the first, wh^-'rivh^ the second, ivh^ the third layer of the root-cap, each layer now consists of two divisions ; 'x y the 

 youngest segments of the substance of the root ; o epidermis ; gf fibro-vascular bundle ; h the part of the root-cap which 

 extends furthest back. 



covered with thick layers of tissue, which it breaks through in its further growth. 

 Hence roots are always endogenous formations, by which character they are dis- 

 tinguished from all trichomes and leaves, and from most lateral shoots. 



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

 Morpliologie der Gewebe. Leipzig 1868, Sect. 5. — Hanstein, Botan. Abhandlungen. Bonn 1870, 

 Heft I. — Dodel, Jahrb. fiir wiss. Bot., vol. VII. p. 149 et s^g.— Reinke, Wachsthumsgeschichte der 

 Phanerogamenwurzel, in Hanstein's Botan. Untersuchungen, Heft III. Bonn 1871. — Van Tieghem, 

 Recherches sur la symetrie de la structure des plantes vasculaires, Fasc. i, La racine, Paris 187 1 ; 

 (also in Ann. des Sci. Nat., 5th ser., vol. XIII, 187 1.) 



^ I choose this expression because it appears also to fit the primary root of the embryo of 

 Vascular Cryptogams. % 



