DAMPING OFF. 243 



for the appearance of the disease after long periods of drought and 

 after the inclement weather of the winter season in some sections. 



Not only does the thicker wall of the oospore offer greater pro- 

 tection against an unfavorable environment, but the protoplasm 

 undergoes a marked change before it finally enters upon this en- 

 forced period of rest. This change is practically a metamorphosis, 

 the complete nature of which we do not understand. Among 

 other changes there is probably a change in the molecular or 

 physical structure of the protoplasm by which a large amount of 

 a fatty substance is separated and forms a very large globule and 

 sometimes other smaller ones which occupy a large part of the 

 space of the oospore. The protoplasm thus becomes transformed 

 into a state highly resistant to outside conditions and incapable of 

 growth for a long period, even though the environment may be 

 most favorable for growth. The period of rest lasts for several, 

 four to five, months. They will resist freezing for weeks, fol- 

 lowed by drying, without injury. 



Propagative organs. Organs of another kind than oogonia and 

 antheridia are developed on the mycelium. The function of these 

 is chiefly for the immediate and rapid propagation of the numbers 

 of the parasite. The organs are, like the oogonia, either terminal 

 or intercalary swellings of the hyphae, and at first do not differ 

 materially from them before the defferentiation of the egg cell 

 and antheridium. These organs are exactly alike in form but 

 differ in the discharge of their functions and are termed respec- 

 tively, conidia, resting conidia, and zoosporangia. 



Conidia. The conidia measure about the same as the oogonia 

 and when fresh water is added to them they will germinate im- 

 mediately after maturity, which is attained upon reaching their 

 full size. 



Resting conidia. These are conidia which do not germinate 

 immediately and acquire a somewhat thicker wall than the con- 

 idia. They pass through a period of rest before germinating. 

 They are identical in form and size with the conidia. They are 

 capable of growing after being frozen, and after drying, and serve 

 in this way much the same function that the cospores do in that 

 they tide the fungus over quite long periods which are unfavor- 

 able for the growth of the plant. 



In germination the conidium thrusts out, by an extension of 



