290 Bulletin 145. 



has had a place in economic literature. Hereafter, at least until 

 unsusceptible and otherwise satisfactory varieties are introduced, to 

 the successful orchardist a knowledge of scab is as essential as a 

 knowledge of pruning. 



2. Special Characters of the Fungus. 



{a.) Microscopic appearafice. — The olivaceous growth on the fruit, 

 leaves, and twigs is largely made up of short erect threads somewhat 

 uneven at their tips. These threads produce the spores or repro- 

 ductive bodies. A thin section through a scab spot on the fruit 

 examined under the microscope shows what I have represented in 

 figiu'e 169. Over the entire surface of the pear, as of plants in 



170, — Germinating spores of the pear-scab fungus. 



general, there is a definite outer layer of cells which we call the 

 epidermis, as shown at e. The outer wall of this epidermal layer 

 usually becomes thickened into a very tough resistant cuticle, r, 

 figure 1 69. Inside of the epidermal layer of the pear we find the 

 pulpy cells and the gritty cells of the fruit without special regularity, 

 as at g. In the diagram it will be seen that the mat-like mass of 

 fungous threads is developed largely just beneath the epidermis,* 



*It will be seen that the above description of the location of the hyphse 

 is somewhat different from that given by several authors. Sorauer* states 

 that the stroma occupies mainly the epidermal cells in apple scab. Ader- 

 holdf in his interesting paper on the Fusicladia finds the hyphce always 

 subcuticular at first, and mainly subcuticular throughout. My sections, 

 made from pear fruit only, show that the principal stroma in this case is 

 undoubtedly at first subepidermal. The sections studied were made from 

 material imbedded in parafiine, and they were stained with the Ehrlich- 

 Biondi-Heidenhain mixture. 



* Pflanzenkrankheiten, Zweite auflage, II., 394. 



t Aderhold, Cf. Bibliography. 



