THE STEMS 95 



in the upper internode, and the bands are closer together there, giving 

 this portion of the straw an almost uniformly green tint ; in the three 

 lowest internodes the tissue is only found in the upper parts, and is much 

 reduced, or entirely absent, at the bases. 



(4) The colourless ground parenchyma extends from the hypoderm 

 to the centre or to the hollow pith-cavity found in most wheats. It is 

 composed of thin-walled, finely-pitted cells polygonal or rounded in section, 

 those bordering the stereome being long and narrow (350 ^ long, 35 p in 

 diameter), nearer the centre they are shorter and wider (150-250/7, long 

 and up to 100 /u, in diameter) ; they soon die and lose their cell-contents. 



In the lowest internodes the walls of the parenchyma become thick- 

 ened and lignified ; the tissue then materially assists the hypodermal 

 stereome in strengthening the base of the straw. Very thick- walled ground 

 parenchyma is found also in the lower part of the solid diaphragm between 

 the internodes, forming a lining to the upper part of the hollow pith-cavity. 



(5) In a transverse section through the hollow internode the vascular 

 bundles are found distributed symmetrically in the wall of the straw. Im- 

 bedded in the hypoderm is a ring of very small bundles separated from 

 each other by somewhat wide intervals, each bundle being placed between 

 two green lines of assimilating tissue (a, Fig. 72). 



Nearer the centre, in the soft parenchyma, are the large bundles of the 

 stem, arranged more or less regularly in a ring, with an occasional bundle 

 of intermediate size close to the inner edge of the hypoderm. 



The large bundles are of the usual closed collateral type, with the 

 xylem directed towards the centre of the stem, its chief vessels arranged 

 in the form of a V . 



The protoxylem forming the apex of the V consists of one or two 

 annular or spiral vessels ; two pitted vessels with wider lumina are 

 situated on the right and left limbs of the V , the space between the chief 

 vessels being occupied by small tracheids. 



The phloem of the bundle lies between the open limbs of the V and 

 extends slightly beyond it. It is composed of sieve-tubes with their 

 companion-cells. 



Surrounding each large bundle is a sheath of fibrous stereome, which 

 in the case of the small hypodermal bundles is not differentiated from the 

 stereome in which they are imbedded. 



The bundles of the leaf-sheath curve inwards into the node and pass 

 through the latter as separate isolated strands without fusing with each 

 other or with the neighbouring bundles which have come from the inter- 

 node above. 



In the leaf-sheath itself all are of somewhat similar diameter, but in 

 traversing the node approximately half of them become very much thick- 

 ened, and alternate in more or less regular order with the other half, 



