727 



Fig. 162, J illustrates the cross-section through a normally dried oat 

 leaf from a locality free from smoke. In the cross-section the leaf appears 

 as thin as a cord because the mesophyll (V) is approximately empty and the 

 cell walls have collapsed. The leaf does not shrivel in the same way around 

 the larger vascular bundles because the strong layers of bast serve as stiffen- 

 ing; they look like knots in the cord-like form. In spite of the great drying 



163. Leaves of a i-ed beech, affected by sulfurous acid, 

 and Reuss.) 



(After V. Sclirr.flor 



of the leaf, the epidermis retains its natural height and at most turns a pale 

 quince yellow like the bast cords, and is thus distinguished from that injured 

 by acids. Fig. 162, 4 is a magnified group from Fig. 162, 5. E indicates 

 the epidermis ; below this, the collapsed mesophyll cells in which the scanty 

 cytoplasmatous remnants of the cell content have been made recognizal)le 

 by soaking tlie section in water. Also in the oat leaf which has matured 

 slowlv in continued wet weather the part injured by acid differs in color 



