>S4 



Bulletin 276. 



unfavorable to the host. A tliickeriini,' of the leaf results. The numerous 

 tiny green bodies (chloroplastids) wliich in the healthy leaves fill most 

 of the cells and take an important part in the manufacture of food in 

 the tree, are not found in the affected cells. This accounts largely for 

 the yellowish color of the diseased parts. 



The ramifying mycelium finally pushes up (Fig. 81) between the 

 epidermal cells and grows out tinder the thin transparent cuticle or 

 protective coat of the upper surface of the leaf. This subcuticular my- 

 celium branches and spreads in all directions. From this mycelium the 



HEALTHY 



D/Sf:/i5£D 



ce//s f/7/ed 

 l/i//fh fhe 

 cfy/orop/ast/c/s 



ch/orop/ast/c/s 

 'destroc/ed 





/oiA^er ebidcrm/s 



Fig. 82. — Cross section through a pencil leaf along tlte line of diseased and healthy 

 tissue. Note the increase in size and number of cells in the diseased portion and 

 the marked absence of the leaf-green bodies or chloroplastids. It is the increase 

 in the number and size of the long cells just imder the upper epidermis that causes 

 the thickening and cnrli)ig of the leaf. (Drawing with camera lucida.) 



asci are now produced (Fig. 81). These stand up in a layer all over the 

 diseased surface, pushing off the cuticle and giving the leaf a frost)^ white 

 appearance. In each of these asci are produced eight globose spores. These 

 frequently increase greatly in number by budding so that an ascus may 

 contain many small spores instead of the eight relatively large ones. The 

 upper end of the ascus now opens and the spores are dispersed. In this 

 State the maturing and dispersal of these spores occurs usually during 

 May. What becomes of them, how they pass the summer and following 

 winter is not known or at best only surmised. Since only the first few 

 leaves that develop are affected it seems that these spores play little or 

 no part in spreading the disease during the spring in which they mature. 



