BICOLLATERAL BUNDLES. 12? 



rich in starch, the so-called starch sheath, is a cortical paren- 

 chyma, which in the fresh stem contains chlorophyll ; and then 

 an interrupted ring of quite typical shining colourless collen- 

 chyma. At the interruptions in the collenchyma ring the inner 

 chlorophyll-containing cortex extends to the epidermis, which has 

 stomata at these places, by means of which gaseous interchange 

 with the inner cortex is maintained. In the interior the stem is 

 hollow. Treated with iodine the starch sheath shows up very 

 clearly, and the protective ring of sclerenchyma is very deeply 

 stained. The individual vascular bundles have no protective 

 sheath, and are not sharply delimited from the surrounding 

 tissue. The parts appertaining to the vascular bundles are, how- 

 ever, more deeply stained than the ground-tissue. If we imagine 

 the inner bast removed, the figure approaches as nearly as 

 possible to the structure of the dicotyledonous bundle of Eanun- 

 culus and Chelidonium. 



Choose a vascular bundle in which are two exceptionally large 

 ducts, as fully developed as possible. If we commence our ex- 

 amination at the outer limits of the inner bast, we shall see at 

 the inner edge of the xylem the crushed protoxylem, scattered 

 in the thin- walled, unlignified, sometimes chlorophyll-containing, 

 xylem parenchyma. Then follow, outwards, annular and spirally 

 thickened vessels (or vascular tracheides), successively increasing 

 in diameter ; then spiral vessels with close spirals, then, in the 

 median portion of the bundle, pitted ducts, some very narrow. 

 The two large ducts on the flanks of the vascular bundle have 

 bordered pits, and are surrounded by flat wood parenchyma ; 

 similar parenchyma surrounds the median group of narrow pitted 

 ducts. Upon this xylem parenchyma impinge externally the 

 radially arranged cell-rows of the cambium ; these pass over into 

 the active elements of the bast. This latter consists of broad 

 sieve-tubes, each accompanied by one (rarely two) narrow com- 

 panion-cells, and of ordinary bast parenchyma, broader than the 

 companion-cells. This bast parenchyma increases proportionately 

 towards the exterior, where is found the highly refractive proto- 

 phloem. The inner bast string is constructed exactly as the 

 outer, but in inverse order. It overlaps the inner edge of the 

 xylem like the horns of a new moon. The two are separated by 

 radially-arranged thin-walled parenchyma, which, however, must 

 not be looked upon as a cambium, since it has early lost its capacity 

 for growth and become permanent. In the sieve-tubes of Cucur- 



