SECONDARY THICKENING. NORMAL DICOTYLEDONS. 527 



(i) Only at the outer limit of the primary bundles, surrounding their phloem- 

 portions (comp. p. 422), and not in the products of secondary growth. This is the case 

 in the stem and branches of Fagus, Betula, Alnus, Platanus, Viscum, Menispermum, 

 Viburnum Opulus, Convolvulus Cneorum, Nerium, Cornus, Punica, Camellia japonica, 

 Drimys Winteri, Ephedra distachya, Abietinese ^, &c. 



(2) Both at the outer limit above-mentioned, and also in the interior of the 

 secondary bast. This latter condition is no doubt the most frequent, especially 

 among woody plants. With reference to the relative amount and distribution of the 

 fibres, it presents very various modifications. 



In the medullary rays fibres are rarely present, e. g. isolated ones in Tilia, 

 comp. Fig. 212. The principal forms of their distribution in the bundles are the 

 following : — 



{a) Concentric layers or rows of fibres alternate regularly with similar layers of 

 soft bast. The layers of both kinds in neighbouring bundles fit approximately, 

 though not always quite exactly, one on another, so that they form annular zones, 

 extending round the whole stem, and interrupted by the medullary rays. 



This phenomenon occurs with especial regularity, as already mentioned at 

 p. 522, in the Cupressineise and many Taxinese, where every fourth secondary 

 tangential row of cells becomes a uniseriate zone of fibres, which separates two 

 triseriate zones of soft bast from one another. Comp. Fig. 211. 



Among the Dicotyledons no such strict regularity exists. The fibrous layers 

 always consist, on the average, of two or more tangential rows, and the number of 

 these rows changes in the same individual, both in the successive annular zones and 

 within each individual portion of the bundle ; as follows from these facts, the thick- 

 ness of the zones of soft bast is also unequal. The conditions in question are also 

 extremely various, according to the species. In the case of many species, however, 

 a regular alternation of concentric zones of fibres and soft bast, of definite average 

 breadth, takes place within the limits of deviation indicated, the original radial and 

 tangential seriation of the fibres of each portion of the bundle being sometimes 

 maintained, as in Vitis, Spiraea ulmifolia, Pterocarya caucasica, and species of Acer, 

 though in most cases their position as seen in transverse section becomes irregular, 

 owing to the displacements due to longitudinal extension (p. 470) : Tilia, Cheirostemon, 

 Sparmannia, Malvaceae, Medinilla, species of Salix, Ladenbergia globosa-, Vas- 

 concella monoica, Guaiacum, and Clematis Vitalba ; comp. Figs. 212 and 214. 



(b) Concentric zones of fibres, alternating with soft bast, may still be dis- 

 tinguished generally, and in places are even regularly arranged; on the whole, 

 however, they are irregular, as they are both interrupted in each bundle by elements 

 of the soft bast, and are also unequal in number and dissimilar in arrangement in 

 neighbouring bundles. This condition, with numerous variations according to 

 species and individuals, and in the average number and breadth of the successive 

 zones, is characteristic of the bast of very many woody Dicotyledonous plants, 



' Compare Hartig. Forstl. Culturpfl. pp. 13, 212, 326, &c. — Hanstein, I.e. p. 21. — Schacht, 

 Der Baum, p. 381. — Von Mohl, I.e. p. 891. 

 ^ Berg, Atlas, Taf. 29. 



