28 BULLETIN" 1074, U. S. DEPARTMENT OF AGPd CULTURE. 



LEAF CHARACTERS. 



The principal parts of the leaves of wheat plants are the sheath, 

 blade, ligule, and auricle. None of these parts usually show differ- 

 ences which are of even minor value for distinguishing cultivated 

 varieties. 



The blades of wheat varieties vary considerably in their dimen- 

 sions, in the shade of green color, and in the angle to the culm main- 

 tained during the successive periods of plant growth. These differ- 

 ences, however, are apparent during only a short period. As the 

 plant matures, the blades dry and frequently break off. Practically 

 all cultivated varieties normally have three leaves, although this 

 sometimes varies under unfavorable or very favorable conditions. 

 In this bulletin very little use is made of leaf characters. A few 

 varieties are noted as having especially broad or narrow blades or as 

 being pubescent. 



Koernicke and Werner {133) and others have described the color 

 of the blades of both the seedlings and the partly grown plants. 

 This also was attempted in the present studies, but the differences 

 were found to be so slight and undependable that no definite classes 

 could be established by using the character. No two persons can 

 agree as to the various shades of green shown by the blades of wheat, 

 even when a standard color chart is used. The color varies with the 

 condition of the plant as affected by the temperature, the soil mois- 

 ture, and the soil solution. The appearance of the color is changed 

 by the character of the venation and of the blade surface. The plants 

 appear to have a different color in the sunlight than in the shade, 

 and the value changes also according to the position of the observer 

 with regard to the direction of the rays of the sun. In general, the 

 Crimean wheats have dark-green blades, while all durum varieties 

 have blades with a light-green color. 



The blade widths are mentioned in describing only a few varieties, 

 because nearly all varieties are very much alike in this character. 

 The Crimean wheats are distinctly narrow leaved, while varieties 

 like Sol and Red Russian have wide leaf blades. In America the 

 winter varieties having the narrowest blades usually are most winter 

 hardy. The length of the blade has not shown sufficient constant 

 differences for taxonomic purposes. 



The terminal leaf of different varieties of wheat is sometimes quite 

 erect and sometimes drooping at various angles. These differences 

 are greatest just previous to the heading period, but frequently are 

 not apparent a few days later. Chiefly because of the instability of 

 this character, it is not used in this classification. 



The sheaths normally inclose about the lower two-thirds of the 

 culm, although in dry seasons the spike sometimes is not entirely 



