102 



The pericycle as in Vhjtocrene consists of a zone of paren- 

 chyma in which groups of fibres are imbedded /? c Figs. 1 and 4 

 of which the much-thickened walls show no sign of lignifica- 

 tion but give a clear violet color with chlor-iodide of zinc. 

 The outer cortex consists only of collenchyma , parenchymatous 

 hypoderm, and the epidermis; the stone-cell-layer and crystal- 

 sacs of Phytocrene failing completely. The periderm furthermore 

 originates entirely in the epidermal layer, developes centripetallj, 

 and shows none of the irregularities so noteworthy in Phi/tocre7ie. 



Having thus surveyed the normally oriented tissues of the 

 fibro-vascular system and the outer cortex, we may at this 

 point conveniently return to the consideration of the pith and 

 the peculiar structures it contains. And here, as will be found, 

 lies the chief anomalous feature of the stem; for* even in a 

 young shoot, of the size indicated, scarcely any of the pith 

 remains in place, the original tissue having been largely de- 

 stroyed , and its cells collapsed by a vigorous secondary growth 

 of phloem. The elements , which compose this secondary tissue , 

 correspond closely in size, nature, and arrangement withtliose 

 of the normally oriented bast. They are without exception 

 soft walled, stand in radial rows, and consist, as will be seen 

 from PL XVIII Fig. 2 of large sieve-tubes and cambiform cells of 

 varying size and shape. The component elements of the sieve- 

 tubes communicate with each other at their oblique ends by 

 series of broadly elliptical sieve -plates arranged transversely 

 on the partition walls. Similar sieve-pits also occur sparingly 

 where two sieve-tubes touch eachother laterally. The sieve- 

 plates in the material examined were frequently covered with 

 callus. 



The origin of this secondary phloem in the pith is readily 

 traceable to two separate strips of cambium, which lie along 

 the sides of the elliptical ring formed by the primary vascular- 

 bundles. These bands of meristem, which give off from their 

 inner surfaces such abundant phloem toward the centre of the 

 stem, produce from their outer surfaces little or no vascular 

 tissue, the entire product consisting as will be seen from 



