2 40 SHEPHERD' S P URSE. 



the stem, and inclosing the inner region of the funda- 

 mental tissue, the pith. The parenchyma rays 

 (medullary rays) left between the bundles may be 

 broad or narrow. The arrangement and course of the 

 bundles depend largely upon the position of the leaves. 

 From each leaf one or more bundles enter the stem 

 and passing downward finally become part of the fibro- 

 vascular zone. Transverse sections of the stem often 

 cut across bundles midway in their course from the 

 leaf to the vascular ring, and they then appear as if 

 belonging to the cortex. The bundles are collateral, 

 with a cambium layer between the xylem and phloem, 

 forming the characteristic open bundle of dicoty- 

 ledons. In Capsella a bundle-sheath arches over each 

 bundle, and frequently becomes continuous around 

 the entire fibre-vascular zone." In the xylem the 

 spiral and annular vessels are the oldest and most 

 centrally placed, the dotted ducts, the largest elements 

 of the xylem, occurring nearest the phloem 15 



The leaf shows the general dicotyledonous charac- 

 ters of more contorted epidermal cells and more num- 

 erous and smaller stomata. The fibro-vascular bun- 

 dles are like those of the stem, tracheides replacing 

 other vascular elements in the ultimate ramifications. 



Capsella is so favorable for the study of the 

 development of the embryo, that this very import- 

 ant subject has been deferred until now. It has 

 already been seen how the asexually produced pollen 

 spore (microspore), after falling upon the papillated 



14 Pointed out by Kamienski, in DeBary's Compar. Anat., p. 415. 



15 For stem structure see Prantl and Vines, Text-book, p. 47 ; Bessey, 

 Bot., p. 438 ; Goodale, Physiol. Bot., p. 119. 



