CHAPTER XVII. 



SECONDARY THICKENING IN MONOCOTYLEDONS 



AND FERN-LIKE PLANTS. 



Sect. 197. Most stems and roots of Monocolykdons show no secondary 

 anatomical changes after the primary differentiation of tissue, with the exception 

 of the formation of a superficial periderm, which often appears, especially in roots 

 and rhizomes, but is by no means general. Comp. Sect. 24, and Fig. 168, p. 360. 

 Its origin and properties are, as far as known, the same as described for the cortex 

 of the Dicotyledons \ 



A cambiogenetic secondary thickening, forming wood and bast, is completely 

 absent in the large majority even of tree-like stems of INIonocotyledons and their 

 roots. After the development of the primary bundle-cylinder the arrangement of 

 tissues within the epidermis or layer of periderm undergoes no further change. It is 

 true that there are statements and controversies upon the point that the internodes 

 of such stems, e. g. of Palms, increase in girth for years after their first differentiation 

 of tissue and extension, a phenomenon which, if true, depends upon an increase in 

 volume of existing tissue-elements, not upon a cambiogenetic secondary formation. 



Cambium and secondary wood and bast appear, as far as at present known, 

 only in the more or less arborescent stems of Aloineae (Aloe, Lomatophyllum, 

 Yucca), of Beaucarnea, and the Dracoeneae (Dracaena, Cordyline, Aletris, &c.); in 

 tubers of Dioscoreacea? ; species of Dioscorea, Tamus, Testudinaria ; and the roots 

 of Dracaenese ^. 



The primary arrangement of the above-named sle?ns is according to the Palm 

 type (p. 262). When it is at least so far advanced that all the primary vascular 

 bundles have been begun, and are in course of development, the cambial layer 

 appears : in a number of species, as Yucca aloifolia, Calodracon Jacquini, Aloe plica- 

 tilis, and Beaucarnea tuberculata, it appears immediately below the apex of the stem, 

 even before the differentiation of tissues in the transverse section at that point is 

 complete; in most Dracaeneae on the other hand — D. reflexa, marginata, Aletris 

 fragrans — it appears in regions of tlie stem of considerable age, which have long 



' Compare Sanio,- Pringshcim's Jahrb. II. p. 66. 



^ Treviranus, Physiol. I. p. 197. — Meneghini, Ricerchc, /. c. ; compare p. 263. — Unger, Di- 

 cotyledonenstamm {Ic , compare p. 249), p. 46. — Schleiden, Grundziige (3 Aufl.), II. p. 159. — 

 .Scliacht, Lehrb. I. p. 329 et passim. — Nageli, Beitr. I. p. 21. — Miilardet, Anatomie, &c., des Yucca 

 et DraCcxna. Mem. Soc. des Sc. Nat. de Cherbourg, tom. XI. — Rauwenhoff, Bydr. tot de Kenntn. v. 

 Dracaena Draco. Amsterd. 1864 (nach Wossidlo). — Wossidlo, Ueber Wachsth. u. Struct, d. Drachen- 

 baume. Progr. Breslau, 186S (sec he:e the older literature).— Falkeuberg, Vegetationsorg. d, Mo- 

 nocotyledonen. Stuttg. 1876. 



