SECONDARV THICKENING. NORMAL DICOTYLEDONS. 497 



suaveolens, and other Proteaceae, species of Ficus, Cordia pallida and many others, 

 every annual ring has several concentric bands of metatracheal parenchyma. 



According to Sanio, parench}TTia always occurs scattered between the tracheides 

 in Dicotyledonous woods, \\'ith the exception of Casuarina, where it only occurs in the 

 metatracheal arrangement, and of Rosmarinus officinalis. It is absent between the 

 typical woody fibres, according to Sanio, with the exception of Edwardsia grandiflora, 

 Ulex europaeus, Celtis austrahs, Olea europsea, and further of Hibiscus Rosa sinensis, 

 where it actually- forms tangential bands between the fibres. In Tamarix gallica it 

 occurs even between fibrous cells which contain starch. 



In some few cases of the wood of roots, which consists chiefly of masses of 

 parenchyma, the latter are the seat of passages containing secretions : root of Inula 

 Helenium'and Opoponax Chironium; Trecul's statement respecting Oenanthe crocata 

 perhaps also refers to this subject ^. 



For the intermediate fibres, the same rules hold good as for the bundle-paren- 

 chyma, because they occiu" either accompan}ing or representing the latter. 



Although the woody fibres may occiu in all the layers of the annual ring, they 

 are present in especially large nimibers in its central portion, in the case of hard 

 woods. They here usually form the fundamental mass, in which the other elements, 

 particularly vessels and parenchyma, are imbedded; in many woods, e.g. Robinia and 

 Gleditschia, they occur only in the central portion of the ring, and are absent in the 

 spring and autumn wood. In woods which are mainly parench^-matous (as Bombax 

 and Cheirostemon), and in that of Avicennia, which chiefly consists of vessels, the fibres 

 are only imbedded in small groups, or singly, between the elements of the funda- 

 mental mass. 



For the fibrous cells, whether septate or non-septate (Berberis, Clematis, Vitis, 

 Tamarix, Punica, &c.), the same rule of distribution holds good as for the woody 

 fibres. As shown by the above statements as to their occurrence, the two elements, 

 which are similar in form and in the structiue of their walls, may mutually represent 

 each other, indeed it is doubtful whether the case mentioned under 7, of the simul- 

 taneous presence of both, ever occurs. 



The tracheides may likewise form by themselves the fundamental mass of the 

 wood, representing, as it were, the two tissues last mentioned ; this is the case in the 

 combination mentioned above under i, e.g. Pomaceae, Camellia, &c. They then 

 always belong to the ' fibriform ' category, resembling woody fibres in shape, and in 

 the character of their walls. Where, en the other hand, they occur in conjunction with 

 fibres and fibrous cells, they are present chiefly in the neighbourhood of the vessels : 

 and in fact they occur next them in small numbers, and isolated, when the latter are 

 scattered singly or in small groups in the annual ring, and are all alike (e. g. Punica, 

 Fuchsia globosa, Ceratonia, and Nerium). When, on the other hand, two kinds of vessels, 

 distinguished by their size, and usually also by special structure, are present, then the 

 tracheides accompany the small ones, rarely the large ones also (Quercus peduncu- 

 lata, Castanea vesca, and Periploca). According to the mode of occurrence of the small 

 vessels, they are then either distributed with the latter in the whole annual ring (Ulex 



* See Berg, Atlas d. pharm. Waarenk. Taf. X. 

 "^ Trecul, /. c, ; compare p. 448, 



Kk 



