SECONDARy CHANGES OUTSIDE THE ZONE OF THICKENING. 539 



of transformation. Until this occurs the external cortex appears traversed by the 

 altered bands, which I have already termed elsewhere ' dilatational bands'.' 

 Their occurrence indicates an unequal participation of alternate bands of the 

 cortex in the dilatation, and this will find its explanation in dissimilar mechanical 

 conditions, which are still to be more minutely studied. In Tilia the first dilatational 

 bands of the external cortex correspond exactly to the most dilated medullary rays ; 

 in Acer striatum they often coincide with the intermediate spaces between the outer- 

 most bundles of bast-fibres, which are filled with parenchyma, but they do not 

 occupy this position exactly or constantly. 



In the peripheral layers corresponding to the original collenchyma the con- 

 nection of the cells reipains close and approximately unbroken, if the local in- 

 terruptions due to the lenticels, to be described below, be left out of consideration. 

 In the inner, lacunar, portion of the external cortex the cavities originally existing 

 grow in the direction of the dilatation. Further, and often extensive, interruptions of 

 continuity may be produced both in the region indicated, and also, though less 

 frequently, in the more deeply situated layers of the bast, as the effect of the tension, 

 distortion, and pressure which the tissues undergo duriilg growth in thickness. 

 In proportion to the originally lacunar structure of the external cortex, it therefore 

 becomes traversed to an increasing extent, as dilatation proceeds, by broad, crevice- 

 shaped cavities, and is often split up into irregular concentric lamellae, e. g. species of 

 Prunus and Pyrus, ^sculus, &c. A similar subdivision into lamellae is shown, for 

 example, by the bast-zone in Berberis and Mahonia. In addition to this, radial 

 cracks appear in the bast in several cases, especially in the species of Prunus ; they 

 are situated along the lateral boundaries of the medullary rays, and increase in width 

 towards the outside, because the cells of the rays, while usually remaining in con- 

 nection among themselves, grow in breadth to a much smaller extent than the rest 

 of the cortex, and a severance therefore ensues in the lateral limiting surfaces 

 •between rays and bundles. 



Secondary Sclerosis is the name given to the phenomenon that individual cells, 

 or definite groups of cells of the parenchyma, assume the sclerenchymatous character 

 after the differentiation of tissues is complete, often thickening their walls to an 

 immense extent during lignification, at the expense of the internal cavity. During 

 this process they sometimes approximately maintain their original form (short 

 sclerenchyma, stone-sclerenchyma), while they sometimes undergo considerable 

 changes of shape and size (many-armed stone-sclerenchyma). 



These phenomena occur principally in very persistent portions of the cortex 

 of woody plants, in some to a very large extent, in others to a less extent or not at 

 all. The actual structure and the strength of the older cortices are in a high degree 

 influenced by this process. Conspicuous as these structures are, and often as they 

 have been described, yet the history of their origin is still much in want of more 

 accurate investigation. 



The limiting zone between the external cortex and the bast-layer is, in the 

 first instance, the special region of these stony formations, and is also usually first 

 in point of time. In numerous woody plants, which in this zone form sclerenchymatous 



' Botan. Zeitg. 187 1, p. 605. 



