512 



NATURAL HISTORY OF PLANTS. 



of them is not noticeably distinct from that of Dicotyledons generally, 

 but in several species certain tissues are exceptionally modified, 

 especially in the liber and woody system. " In the tribe Urerece, for 

 instance, which gives us an exceedingly interesting study in this 

 respect, the medullary rays are much prolonged vertically, without 

 anv consequent change in the form of their cells, and are often seen 

 to constitute real septa between the fibrovascular bundles." 1 The 

 vessels are very variable in calibre, most voluminous where the wood 

 is soft. The dots too, rounded or more or less transversely elon- 

 gated, are larger as the wood is less compact. Some Nettles have 

 been said to lack medullary rays. 2 Guillard 3 thinks the woody fibres 

 remarkable " in the thinness of the walls, retained even in the adult 

 branches, in the regularity of their prismatic quadrangular form, 

 and in their equality in thickness, about T w of a millimetre." The 

 same author notices that the trachea? of the medullary sheath are 

 sometimes few and very large, approximated into bundles, sometimes, 

 as in certain species of Urtica and Pilea, " grouped in no apparent 

 order." He distinguishes the medullary rays as " extraordinary for 

 the height of their cells." But all authors' 1 agree in recognising that 

 it is especially in the organization of the liber that the Urticacece are 

 distinguished from the rest of Dicotyledons by a striking character. 

 In fact, the cortical, singly very long and very fine, adhere to one 

 another end to end, though not fused ; this renders them textile and 

 of industrial value. Moreover, it is easy enough to make out that 

 the fibres are so arranged inside the bark as to form concentric 

 zones, and that each fibre is separated from its collateral neighbours 

 by one or more rows of cells ; the fibres of the outermost layer are 

 almost scattered through the cellular tissue of the middle layer of 

 the bark." 5 The suberous layer is generally ill developed. 6 The 

 wood may present peculiar features in some trees that grow to a 



1 " I found this arrangement in a woody 

 Pilea; it, no doubt, is also present in Ela- 

 tostema." (Wedd., op. cit., 8.) 



2 Chatin, Anat, Comp. des Veg., livr. iii. 9 

 (not.). 



3 Loc. cit., 314. The author gives in the same 

 work (p. 312) a table of the general structure of 

 the stem, and represents that of Parietaria 

 (Gesnouinia) arborea (figs. 15, 17) ; he also 

 describes and figures the structure of the leaves, 

 the starch-granules, crystals, raphides, &c. 



4 Wedd., in Ann. Sc. Nat., ser. 4, vii. 309. — 

 Guill., loc. cit., 312, 313. — Keissek, in Denk. 

 d. K. Akad. Wiss. Wien, vi. — Oliv., Stem in 

 Dicot., 33 (in Nat. Hist. Rev., iii. 252). 



5 In fine, the liber closely resembles that of 

 Hemp, and has nearly the same properties. 



6 "The suberous layer has usually from four 

 to eight concentric zones. This is as many, or 

 more, than we find in most other orders." 

 (Guile., loc. cit., 315.) 



