498 SECONDARY CHANGES. 



europaeus, and Rosmarinus), or limited to its outer portion (Morus alba, Broussonetia, 

 Catalpa, Paulownia, Sophora japonica, Gymnocladus canadensis, Robinia pseudacacia, 

 Corylus, Carpinus, and Ostrya).— Secondly, the tracheides are in many cases most 

 abundant in the outer part of the annual ring, or confined to it, even apart from their 

 connection with particular vessels. In the annual ring of Ribes nigrum, Syringa 

 vulgaris, Ligustrum vulgare, Euonym'us europaeus and latifolius, they successively 

 increase in frequency towards the outer border of the annual ring, until they form the 

 fundamental mass in which isolated vessels and fibres, or fibrous cells are imbedded ; 

 while in the inner portion of the ring the fundamental mass consists of fibres or 

 fibrous cells, and the tracheides occur isolated, side by side with the vessels. Or the 

 tracheides occur only at the outermost, autumnal limit of the annual ring, while the 

 latter otherwise only contains vessels of one kind, as in Tilia, Salix hippophaefolia, 

 acutifolia, Populus tremula, pyramidalis, Rhamnus Frangula, Juglans regia, cinerea, 

 Pterocarya, Diospyros virginiana, Betula alba, Alnus glutinosa, Laurus nobilis, Cam- 

 phora ; Acer pseudoplatanus, platanoides, campestre ; Sambucus nigra, and racemosa. 



Sect. 150. Certain phenomena which depart in some degree from the usual 

 rules, but belong nevertheless to the normal Dicotyledonous type, are here to be 

 mentioned as special cases by way of supplement ; they chiefly concern the wood of 

 species or families which are distinguished by special adaptations, and by peculiarities 

 of form correlated with these. Most of these phenomena still require more minute 

 investigation, for which the following paragraphs will only contain indications. 



We have, first, to return to the woods enumerated on p. 458 and p. 492, which 

 are destitute of medullary rays. In those cases where only the primary large medul- 

 lary rays are absent, while small ones soon appear, as Ephedra, Cobaea, and no 

 doubt Xanthosia also, the above general rules for the structure and distribution of 

 the organs hold good for the main mass of the wood. As far as the absence of the 

 medullary rays extends in the cases mentioned, and in cases of their entire sup- 

 pression throughout the whole secondary wood, the main and fundamental mass, with 

 isolated exceptions to be mentioned below, consists of thick-walled fibrous cells or 

 fibres— the latter being gradually developed from the former: these are usually 

 elongated, though short in Echeveria pubescens, and have sharply pointed ends and 

 a regular radial arrangement Leaving out of consideration the xylem portions of 

 the original leaf-trace bundles, which project into the pith, and belong to the medul, 

 lary sheath to be described below (Sect. 152), the vessels— perhaps also series of 

 tracheides of otherwise similar structure — and groups of bundle-parenchyma are 

 inserted in the fundamental mass in the more strongly developed wood. 



The grouping of the vessels and of the parenchyma, and the quantity and 

 quality of the latter, vary according to the particular case. In the agrial stem of 

 the Crassulacea'^ investigated, which has a somewhat strongly developed ligneous 

 body, the parenchyma consists of elongated thin-walled cells, which remain 

 unlignified, and accompany the vessels in longitudinal rows. The vessels and 

 parenchyma are either quite isolated and scattered in the mass of fibres, at most 

 two or three occurring together— Sedum maximum ; or some of them form larger 

 groups, consisting of as many as twelve elements— S. populifolium, and Echeveria ')i^ 



' Compare Brongniart, Arch, du Museum d'Hist. Nat. torn. I.— Regnault, /. c. 



