THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



621 



of large intercellular cavities, forming, no doubt, an aerating system. These 

 are called the vallecular canals, as each lies directly below a stem groove 

 and therefore close beneath the assimilating tissue. 



Cortex 



Vallecular canal 



Metaxylem 



Endodermis 'C^^^I'TV rj 

 ""^ '....'■ 



Carinal canal ^' 



Pith 



Fig. 630. — Equisetum arvense. Transverse section of 

 the stem showing canals and vascular strands. 



The cortex is bounded on the inside by an endodermis with well-marked 

 Casparian bands on its radial walls, and within it lies the pericycle, forming 

 the external laver of the stele. The stele itself consists of a single ring of 

 vascular bundles, surrounding the large central cavity of the stem. This 

 cavity occupies the position of the pith, which is present in the youngest 

 internodes but soon breaks down leaving an empty space. 



The general distribution of tissues in the rhizome is similar to that in the 

 aerial stem, but the cortex is simpler, without any assimilatory tissue. There 

 is an outer layer of dark-coloured sclerotic cells (Fig. 631). 



The vascular bundles in the ring are widely separated by broad bands of 

 parenchyma. Each bundle is collateral, that is, it has the xylem on the inner 

 and the phloem on the outer side. The xylem has the shape of a " V," with 

 the point, which consists of protoxylem, inwards (Fig. 632). This point is 

 marked by another intercellular canal, the carinal canal, so called because 

 it lies directly under one of the external ridges of the stem. The protoxylem 

 elements are few and small and adhere loosely to the sides of this canal. 

 The metaxylem forms the two limbs of the " V," but the two groups of cells 

 composing it are separated from the protoxylem by several cells of xylem 

 parenchyma. The phloem lies between the limbs of the xylem and is 

 composed of phloem parenchyma and sieve tubes, with sieve plates on their 

 oblique, transverse walls. 



The separation of protoxylem and metaxylem is more than merely 



