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 foKar 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. 121.— Longitudinal section through a somewhat older primary root oi Marxilea salvatrix ; the apical cell ; wAlx 
WÄ2 the first, lult^-k-iuh^ the second, -wh^ the third layer of the root-cap, each layer now consists of two divisions j/ the 
youngest segments of the substance of the root ; o epidermis ; g f 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. 
^ Nägeli und Leitgeb in Nägeli's Beiträgen zur wissen. Bot,, Heft IV, 1867. — Hofmeister, 
Morphologie der Gewebe. Leipzig 1868, Sect. 5. — Hanstein, Botan. Abhandlungen. Bonn 1870, 
Heft I. — Dödel, Jahrb. für wiss. Bot., vol. VII. p. 149 et seq. — 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, Fase, i, La racine, Paris 1871 ; 
(also in Ann. des Sei. Nat., 5th ser., vol. XIII, 18 71.) 
^ I choose this expression because it appears also to fit the primary root of the embryo of 
Vascular Cryptogams. 
