THE STEMS 



93 



are about 4 /LI in diameter, those covering the green assimilating tissue 

 being wider (10-15 /z in diameter) with more sinuous walls. 



The walls are all pitted and extensively thickened, and on the parts 

 not hidden by the leaf-sheath a cuticle is present, the surface of which 

 in the case of glaucous-stemmed varieties is covered with minute flaky 

 particles of wax. 



(2) The hypoderm is a strong elastic cylinder of mechanical tissue, 

 consisting of lignified fibres with narrow lumina and strong walls about 

 4 \i thick : in a transverse section of the straw it appears as a continuous 

 zone of cells immediately within the epidermis, and is of variable thick- 

 ness, being more or less wavy in outline on its inner side (Figs. 72, 73). 



From the measurement of a full-grown straw of " Squarehead " wheat 

 given on p. 64, it is seen that the average thickness of the zone is about 



a 



FIG. 72. Transvermr section of portion of the culm (lower part of the upper internodc) 

 ( x 70). ii. Small vascular bundle in the hypodenn (A) ; b, large vascular bundle ; c, 

 band of chlorophvllous tissue. 



the same in the 2nd, 3rd, 4th, and 5th internodes ; in the upper internodc 

 it is slightly stronger, and reaches its greatest development in the lowest 

 section of the straw. 



A similar relationship in regard to the thickness of this zone of sup- 

 porting tissue is seen in each of the separate internodes, being stoutest in 

 the basal and thinnest in the middle portions. 



(3) The assimilating tissue of the stem resembles that of the leaf, 

 consisting of delicate parenchymatous cells, which in longitudinal view 

 are irregular in form, but in transverse section are almost circular and 

 about i2/i in diameter (r, Fig. 73). It is imbedded in the sclerenchyma- 

 tous hypodenn, but a portion of its outer face is always in immediate 

 contact with the epidermis, through the stomata, by which it is brought 

 into communication with the atmosphere. 



The tissue is arranged in parallel bands, which appear as narrow green 

 strips along the stem, with lines of colourless stereome between. The 



