122 THE AMERICAN MONTHLY [May, 



girdling it. The supply of nutriment thus being cut oft' from the 

 terminal portion of the twig, death of that portion followed. 



Seated in the original diseased areas, whether extending partly 

 or entirely around the stem, are minute black elevated points 

 which can be seen with the unaided eye. Figure i a and b rep- 

 resent portions of twigs, a with the diseased area extending partly 

 around the stem, while b is girdled. In the diseased areas are 

 shown these small black elevations. The disease is produced by 

 a fungus which grows within the stem. These black elevated 

 points are centres where pustules of the fungus are developed 

 containing its reproductive bodies, or spores. 



Section Across one of the Pustules of the Fungus. — 

 Figure 2 represents a very thin section across one of these pustules, 

 magnified to show the structure of the pustule and the form of 

 the spores. The growth of the pustule has ruptured the epider- 

 mis of the stem. In the opening between the broken parts of 

 the epidermis are shown some of the spores. Beneath these are 

 numerous parallel short fruiting threads, or basidia of the fungus, 

 at the end of which the spores are developed. The basidia can 

 be seen to arise from the blackened stroma of the fungus, which 

 consists of quite a compact association of irregular cells. The 

 black color of the pustules comes from the stroma. 



Method of Separation of the Fungus. — To study the de- 

 velopment of the fungus it was necessary to separate it from other 

 common forms of fungi as well as bacteria which always find a 

 lodgment in and upon dead plant tissue. Since all these forms 

 are microscopic the separation involves a method of procedure 

 familiar only to specialists, and as such beautiful results were 

 reached in the separation of this fungus it suggested a graphic 

 presentation of the method in connection with the study. The 

 method used was the same as that which Koch developed so ad- 

 mirably for the separation of bacteria, and consists in the dilution 

 of the organisms in several quantities of a warm liquid substance 

 which, when spread out in a thin layer and cooled, solidifies and 

 holds each germ firmly fixed at one point in the dilution. This 

 substance is usually some gelatinous base, as gelatin, or agar- 

 agar, containing beef broth and peptone to furnish food for the 

 organisms. In a few days after cooling the dilutions in the thin 

 layer each germ by growth has produced a colony which can be 

 seen with the unaided eye. 



Three glass tubes containing a small quantity of liquid nutrient 

 agar-agar were placed in the water bath at 43'' centigrade. This 

 temperature is sufficient to keep the agar liquid, while it is not 

 hot enough to kill the organisms. Now several thin shavings 

 through the fungus pustules on the stem of the privet were trans- 

 ferred to tube No. I. This was shaken gently to distribute the 

 germs evenly through the liquid. Now a small quantity of the 

 liquid in No. 1 containing the germs was transferred to tube No. 

 2, making the second dilution, and from No. 2 to No 3, making 



