1888.] 



MICROSCOPICAL JOURNAL. 



129 



pears as a small dark brown or black spot, growing upon the upper surface 



of the leaf, as shown in Fig. i, at «. 



It grows rapidly, spreading into the 

 surrounding tissue in a dendritic man- 

 ner, until the greater part of the leaf 

 has become infested. A pale-yellow 

 zone surrounds the spot, and, as the fun- 

 gus develops, the' zone increases, until all 

 that part of the leaf not occupied by the 

 fungus has assumed the yellow tinge. 

 Upon making a vertical section of the 

 leaf and placing it under high magnify- 

 ^""^' ing power, much may be ascertained 



concerning the true nature of the disease. The fungus is found to lie upon 

 the epidermis, immediately underneath the cuticle. The mycelium of the 

 young plant, by rapid growth, forms a stratum or layer in this part of the 

 leaf. At various points in this stratum, the spores or reproductive bodies 



are developed from the myce- 

 lium. They push upward in a 

 mass against the cuticle, which 

 soon bursts and rolls back, form- 

 ing a minute ragged cup, as in 

 Fig. 2. Many of these minute 

 cups upon the blackened surface 

 of the leaf cause it to present a 

 rovighened appearance. 



.Shortly after reaching the sur- 

 face, the spores become detached 

 ^^^■^- and are transported by the wind 



or other agents to neighboring rose leaves, where they soon germinate, and 

 thus the disease is rapidly spread. The spores are very minute in their 

 structure, appearing to the naked eye as dust particles. Magnified five or six 

 hundred diameters they are shown to be oblong 

 bodies, constricted in the middle, and are divided 

 into two cells by a transverse partition. Fig. 3 rep- 

 resents these spores as they appear under the mi- 

 croscope. 



From the layer of mycelium, lying between the 

 cuticle and epidermis, short branches (hypha;) are 

 sent down between the outer epidermal cells into 

 the tissues of the leaf, from which the fungus de- 

 Thus far nothing definite has been determined con- 

 cerning the character of this mycelium and the extent to which its hyphae 

 ramify through the leaf tissues. 



The blackening of the leaf does not originate from any coloring matter in 

 the fungus, but is due to an abnormal growth in the epidermis. The epider- 

 mal cells of a mature rose leaf are divided by tangential partitions into two, 

 an inner and an outer cell. In the healthy, growing leaf these cells are filled, 

 mainly with j^rotoplasm, chlorphyll granules, and cell sap, the outer epider- 

 mal cell containing the greater amount of substance. As soon as the fungus 

 begins its growth, the character and appearance of the contents in the outer 

 epidermal cells at once l^egin to change, while that of the inner cell is not 

 perceptibly affected. It very rapidly assumes the appearance of a dark brown 

 granular pigment which entirely fills the cell. This substance gives the dark 

 color to the 'black spot.' Fig. 2, a, illustrates this growth. 



rives its nourishment. 



