KANUNCULUS EEPENS. 123 



that its diameter is greatest at the closing membrane, a kind of 

 chamber being thus formed in the thickness of the wall, so that 

 they might be called " chambered pits ". The comparatively 

 narrow vessels in the median portion of the bundle form a 

 connected group in which wood parenchyma is completely 

 wanting. 



Outside the xylem is to be found a tissue composed of several 

 layers of flat, unlignified, thin-walled cells, arranged in straight 

 radial rows. This is a growing layer, or cambium, of, in this case 

 however, strictly limited activity. This is our first acquaintance 

 with a cambium layer as a constituent portion of a vascular 

 bundle by means of which xylem and phloem can increase in 

 quantity; and from its presence these are called [open bundles, 

 in contradistinction to the closed bundles we haye studied in 

 Monocotyledons. Outside the cambium is the readily recognis- 

 able phloem or bast, which closely resembles that of Zea Mais, 

 and, as in it, is composed of sieve-tubes (v) and companion-cells 

 (shaded in figure) in fairly regular alternation. Most externally 

 a few protophloem elements are found. A sheath (the upper vg) 

 of sclerenchyma fibres surrounds the vascular bundle, and is 

 specially strongly developed, in the form of an arc, outside the 

 bast. At the outer edge of the phloem it is in direct contact with 

 the protophloem, but, as in Zea Mais, a layer of unthickened cells, 

 belonging, however, to the sheath, separates the bast laterally from 

 these thickened elements. Similarly upon the inner or xylem edge 

 of the bundle, the sheath forms a deeper but thinner arc (the 

 lower vg) separated from the wood by unthickened parenchyma. 

 Between the two arcs of the sheath, opposite, that is, to the ends 

 of the cambium layer, the sheath is apparently wanting these 

 are places of passage or transfusion, by which communication for 

 water or food stuffs is maintained between the vascular bundle 

 and the surrounding conjunctive parenchyma. They are com- 

 posed of cells which are more feebly thickened and only slightly 

 or not at all lignified, and show up particularly clearly, therefore, 

 in the preparation if it is examined in chlorzinc iodine. This 

 vascular bundle, that of an undoubted Dicotyledon, is therefore 

 comparable in a quite marked degree, with that of a typical 

 Monocotyledon like Zea Mais. 



Let us now take radial longitudinal sections through the 

 runner, preparing them in the manner already recommended, i.e., 

 by nearly cutting a piece of the runner through at two points 



