SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 537 



widening of their parenchymatous elements, and through them of the whole bundle, 

 takes place here also. This category includes Tilia, and other woody plants provided 

 with a similarly grouped bast. In young shoots of Tilia the zone of wood and 

 bast is traversed by numerous large parenchymatous rays extending to the pith, 

 most of which are uniseriate, but many, e.g. in transverse sections of a branch 

 of Tilia parviflora now before me, the seventh, ninth, or fourth, and so on, are bi- or 

 triseriate ; the latter is at any rate the case at the cambial limit, and sometimes as 

 far as the pith, while in other cases they become uniseriate before reaching the latter. 

 The dilatation begins in the biseriate and triseriate rays; whatever strands and 

 rays lie between them, take, in the first instance, no part in the process. As growth 

 in thickness proceeds, a constantly increasing number of the original uniseriate rays 

 take part in the dilatation. Later on successive small secondary rays are also in- 

 volved in it. When a ray takes part in the dilatation it is, as a rule, bi- or pluriseriate 

 at the cambial boundary. The result of these phenomena is in the first instance the 

 severance of the bast-layer into the often described ' groups of bundles with a 

 wedge-like form widened towards the cambium, as seen in cross-section, and rays, 

 alternating with the former, widened in the opposite direction, and further, the 

 successively occurring subdivision of the first groups of bundles into more numerous 

 narrower ones, separated by rays. The number of groups in a transverse section, 

 rises, for example, in branches of T. argentea now before me, from 45, in an 

 internode fii"™! thick, to 138 in one 28™™ thick. In order to complete this 

 description, which has been given in the first instance with reference to the transverse 

 section, it must be added that the medullary rays' are of considerable height — the 

 larger ones measuring more than a hundred cells — and their ends situated in quite 

 different transverse sections all round the stem ; and that the dilatation begins 

 in every ray about at the middle of its height, and thence advances upwards 

 and downwards. 



Quite similar phenomena to those in the bast of the Lime reappear in many 

 other plants, and in members of diflferent value, e. g. in the stem and branches of 

 Hibiscus syriacus, Pterocarya, and Galipea ofiicinalis, and in the highly parenchyma- 

 tous roots of UmbelliferK (Archangelica, Levisticum, &c.), Glycyrrhiza, and many 

 others ". 



Intermediate cases between the extremes, represented on the one hand by Salix 

 fragilis, Spir^a ulmifolia, and Punica, and on the other by Tilia and Menispermum, 

 would naturally be expected to occur from what has been stated. In these every 

 transverse section shows radial bands of the bast, which are more or less strongly 

 dilated, in various gradations. Examples are afforded by Sparmannia africana, 

 represented in Fig. 2 r 4, p. 528, with numerous strongly dilated rays, and more slightly 

 dilated ones in the divisions limited by them ; further, in very various gradations, 

 by the Quinine barks, Croton Eluteria, Simaruba officinalis, Cinnamomum zeyla- 

 nicum, &c.' 



In the structure of the dilated masses of parenchyma the increase in the 



' Compare e. g. Schacht, Der Baum, p. 19S ; Lehrb. II. p. 50. — Hanstein, Baiimrinde, Taf. I. 

 ' Compare Berg, Atlas, Taf. 37. 6, 8, 9. 

 f Berg, /. c. Tab. 29-38. 



