ROSE CANKER AND ITS CONTROL. 17 



Germination of the Spores. 



The life cycle begins with germination of the spores. The first essential 

 condition for germination is the presence of water. Spores never ger- 

 minate except when they are directly in water. A moist atmosphere is 

 not sufficient. Germination takes place through the production of one 

 or more tubes from each of the two cells of the spore. Usually the tubes- 

 do not start at the same time; one in each cell begins to grow, and this 

 is later followed by another. Four germ tubes to each spore is the most 

 frequent condition, but there may be more or fewer. The tubes may 

 come out from any place on the surface of the spores, as illustrated in 

 Fig. 7. They elongate very rapidly at laboratory temperatures, quickly 

 develop septa, branch repeatedly and soon a mycelium is produced. 



The brown thick-walled cells of the mycelium, which we have called 

 chlamydospores, germinate by the production of slender hyaline germ 

 tubes similar to those of the conidia and under the same conditions. 

 Other detached cells of the mycelium also possess the power of germina- 

 tion. Especially is it common to see germ tubes arising from the cells of 

 the main stem of the conidiophore when detached and kept in water. 

 Such germ tubes usually arise from the end walls of the cells, and may 

 grow directly through one or more old cells before emerging. 



Temperature Relations. 

 The relation of temperature to germination of spores was studied 

 carefully in the hope of evolving some method of control by keeping the 

 rose houses at temperatures which are unfavorable for germination and 

 thus retarding progress of the disease. The general effect of variation 

 of temperature and the maximum, minimum and optimum temperature 

 for germination were determined by the following method : — 



Method. — Viable spores from a young, pure culture were transferred to a drop 

 of water in the center of a glass slide. The slide was supported on two short glass 

 rods in a Petri dish, used as a moist chamber. A few drops of water placed in 

 the bottom of the dish kept the air humid and prevented drying out of the drop 

 containing the spores. The Petri dish was then kept at the desired constant tem- 

 perature in incubator, refrigerator or constant temperature room. Observations 

 were taken and percentages of germination counted at regular intervals. No figures 

 are based on the results from a single slide. Each result tabulated represents the 

 average of several slides. Tests at high or low temperatures were controlled by 

 duplicates at ordinary room temperatures. 



The results of the tests are summarized in Table I. 



