80 CHARACTERS IN THE LEAF. 



Young leaves of Avena sativa (oats) and Setaria glauca (pig- 

 eon grass) quite uniformly twist against the sun, while those of 

 Poa pratensis (June grass) and Panicicm capillare seem 

 indifferent as to the direction in which they twist. The 

 sun does not seem to dictate the direction of the twisting. 



The leaves of grasses generally twist best and with greatest 

 uniformity when young, even though they are much shaded from 

 the rays of the sun. 



Many leaves twist most towards the apex, while others twist 

 most, or entirely, at or near the base of the blade. 



The margins of many leaves grow a little longer than the cen- 

 tral portion, and if the mid-rib is not very prominent, this will 

 produce torsion of the blade. In cases of Indian corn, the mar- 

 gins of the older leaves are often longer than the mid-rib, but 

 there is no torsion. The margins are undulating. With a light 

 mid-rib and stouter margins, the leaves of this plant would show 

 torsion. Probably one reason why most of the torsion is towards 

 the apex of many leaves is because the mid-rib is not very strong 

 at that part of the leaf. 



When young and quite erect, the lower side of many leaves 

 seems to grow a trifle faster than the upper side, and this per- 

 haps tips the leaf over " bottom side " up. 



Duval-Jouve believes that torsion of the blades of grasses 

 depends on the distribution of the fibrous tissue. In dry 

 weather this tissue contracts least, so the blade twists. 



In some the air canals, lacunm, let in dry air, which contracts 

 the delicate cells of parenchyma. The writer has not yet been 

 able to find the reason for a uniformity in the direction for the 

 torsion of the leaves of any species of grass. 



Generic and Specific Characters in the Leaf.— E. 

 Hackel, in his Monographia Festucarum Europcearum, says: 

 '* The histological characters of the leaf-blade unquestionably 

 include those most important for the discrimination of thfr 



