THE STEMS 97 



in diameter and form a conspicuous ring of strands, each of which is 

 flattened laterally, becoming oval or pear-shaped in section (Fig. 75). 



Around the bundle, in its course through the node, is a sheath of 

 parenchyma containing many green chloroplasts. 



The xylem elements of these large sheath bundles are much increased 

 within the node, the vessels being replaced by a great many short, pitted 

 and reticulately thickened tracheids of small diameter distributed 

 irregularly with xylem parenchyma. 



In the lower section the xylem elements more or less completely 

 encircle the bast, but this amphi vasal arrangement is lost after the bundles 

 leave the node, and the collateral form is soon resumed. 



In the internode above and below the node the cauline bundles are 

 of the ordinary collateral type, but within the node they are much 

 changed in structure ; the sheath of sclerenchyma on the pith side is 

 somewhat strengthened and the vessels become more numerous, the 

 typical V-shaped section being altered to a Y, the stem of which is formed 

 of 4 to 6 vessels of more or less elliptical section. Lower, the vessels on 

 the arms of the V and Y are replaced by groups of short tracheids. 



The number of bundles in a straw varies considerably, stout straws 

 of plants grown wide apart possessing more than those of crowded 

 crops ; in the early developed stems of tillered plants the bundles are 

 more numerous than in straws formed later. 



The total number is generally greatest in one of the two uppermost 

 internodes, but sometimes in the lowest, fewer being found in the inter- 

 mediate portion of the straw. 



In the first four internodes there are usually more in the inner zone 

 of large bundles than in the hypoderm, but in the portion of the stem 

 immediately below the ear the reverse is occasionally the case. 



On the following page are given the numbers of the vascular bundles 

 in the internode of the 6 straws of a plant of T. vulgare (Squarehead) 

 from a row of plants grown 3 inches asunder in rows i foot apart. 



The thin rhizomatous portion of the stem between the grain and the 

 tillering nodes is about -6-1 mm. in diameter. Its anatomy is somewhat 

 different from that of the internodes above ground, there being a clearly 

 circumscribed stele or central cylinder surrounded by a soft thin-walled 

 cortex. The epidermis consists of elongated parenchymatous cells 

 about 7 n in diameter, with thin walls ; hairs and stomata are absent. 

 Within, is the cortex of thin-walled parenchyma, generally 6 to 8 cells 

 deep and about -i5--i6 mm. across ; there is no zone of hypoderm, but 

 occasionally one or two very fine strands of stereome are seen just beneath 

 the epidermis. A fairly well-defined endodermis is visible, differentiated 

 from the rest of the cortical tissue by its thick inner walls, which are 

 lignified and show lines of stratification. 



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