THE LEAVES 



49 



sides. They are usually pale green or pinkish in colour, and their margins 

 and tips are often fringed with a few long unicellular hairs (a, Fig. 42). 



As indicated previously, they are not exactly opposite to each other ; 

 the lower one is always connected with the narrower and longer half of 

 the leaf-blade, and as this half alternates from right to left in successive 

 blades, the lower auricle follows the same succession, encircling the sheath 

 and straw from the right and left alternately. 



On young leaves and on those of the lower nodes of the straw which 

 die off soon, the auricles are small, but on the upper leaves they are more 

 strongly developed. The size also varies with the race and variety of 

 wheat, those of T. Spelta and T. dicoccum being the largest ; the free claw 

 in some cases attains a length of 4 or 5 mm. 



The number of leaves on a well-de- 

 veloped straw of the various races of wheat 

 depends partly on the variety and the 

 vigour of the individual plant. In most 

 varieties of T. vulgare as grown in the 

 field, about 70 per cent of the straws 

 possess 6 leaves, 8 or 10 per cent have 

 7, and about 20 per cent only 5 leaves. In 

 varieties of T. turgidum 20 per cent have 7 

 or 8 leaves, about 60 per cent 6 leaves, the 

 rest only 5 or less. 



The total length of each leaf, i.e. the 

 combined length of its sheath and blade, 

 increases from below upwards to the fifth, 

 which is the longest on the straw ; the 

 sixth or uppermost is invariably shorter 

 than the one immediately below it. The 

 average lengths of the leaves (sheath and blade) of the primary straw 

 of the different races of wheats are given below. 



a- 



FIG. 42. 



i L 



Auricle of leaf ; /, ligule 

 (X2). 



AVERAGE LENGTH OF SUCCESSIVE LEAVES IN INCHES 



