74 



ACEOGENOUS OE AOOTYLEDONOUS STEM. 



by their union, and the partial absorption of their transverse walls, 

 form what are called cribriform or sieve-like vessels (vasa propria), 

 thickened bast cells (6 p), and bast fibres {b /). 



Fig. uo. 



In some Thallogens the thallus or frond is supported by a stalk, in 

 which there are concentric parenchymatous circles, with divisions in 

 the form of rays, but no vascular bundles. These appearances are 

 presented by some large antarctic seaweeds (species of D'UrviUsea and 

 Lessonia), and by some lichens, as Usnea. 



Fig. 140. Section of vascular bundle of stem of Bquisetum hyemale x 310. Lacuna 

 or^a cavity, I ; parenchyma, a form of starch cells, p p ; large vessels, Iv ; smaU vessels, sv; 

 bast cells, & p ; and bast fibres, b f; cribriform vessels, c v, formed by united cells, ■with a 

 partial absorption of their transverse walls. — Trams. Boi. Soc. Edin. 



