THE LEAVES 51 



blade of the uppermost leaf is usually 1-2^ inches shorter than its 

 sheath. 



The length of any particular sheath is approximately equal to the 

 arithmetical mean of the sheath lengths of the leaves immediately above 

 and below it, a relationship similar to that which exists between the 

 lengths of separate internodes of fully developed straws. 



The figures for the sheaths of T. turgidum, T. Spelta, and T. compactum 

 are : 



In the other races of wheat mentioned, the agreement between the 

 calculated length and that actually observed is not quite so close, a dis- 

 crepancy which is possibly due to the averages given in the table con- 

 taining measurements of leaves which had not all reached their maximum 

 development or whose normal growth had been disturbed. 



ANATOMY OF THE LEAVES 

 i . The Coleoptile and Prophylk 



The tissues of the coleoptile are of a simple character. Outside is 

 an epidermis of long cells of rectangular outline with thin straight walls : 

 there are no hairs present. One or two single rows of stomata run from 

 the base to near the tip, on each side of, and parallel to the vascular strands 

 within the leaf, the number of stomata increasing towards the apex, where 

 they are crowded together in an irregular manner. 



The few chloroplasts present are found in most abundance in the first 

 layer of cells beneath the epidermis and in the parenchyma surrounding 

 the vascular bundles ; they remain colourless except when the coleoptile 

 is exposed to light. 



In the majority of wheats two vascular bundles are found on opposite 

 sides of the coleoptile. These are simple and straight for the greater 

 part of their length : at the summit they bend towards each other and 



