POTATO-ROT. 213 



and occasionally through the epidermis even of the upper surface. 

 When these branches have reached beyond the surface of the leaf they 

 form several branches, at the ends of which are developed egg- 

 shaped bodies, (Fig. '2) which burst open at maturity, and from 

 them escape from six to fifteen verj' minute and peculiar I)odies, 

 (Fig. 3) of an ovoid form, with two long threadlike organs by means 

 of which they propel themselves through the water, after the man- 

 ner of certain microscopic animals ; in fact so closely do they resem- 

 ble animals in their movements that they have been called zoospores. 

 from a Greek word meaning animal, and spore. After these zoos- 

 pores have moved about in the dew or water which may be held bj' 

 the hairs on the underside of the potato leaf, and which would form 

 an ocean for these minute bodies, they make their way into the 

 interior of the leaf again through the breathing pores. In about 

 half an hour from the time they are first discharged they come to 

 rest, the thread-like organs of locomotion disappear and the zoospore 

 at once elongates into thread-like mycelium, like that from which it 

 originated. The egg-shaped bodies do not always burst and give 

 rise to zoospores, but sometimes fall olF and develop at once into 

 mj'celial threads, and if b}' an}' chance the}' fall near, or upon the 

 potato plant, this mycelium corrodes and pushes its way into the 

 interior of the plant to continue the same round of life as before. 



It has been calculated that one square inch of the under surface 

 of a potato leaf ma}' yield 3,000,000 zoospores, and that in an 

 incredibly short time these will have growai and developed a new 

 generation, so that it is not at all surprising that after the first 

 appearance of the disease, an entire field of potatoes is so quix;kly 

 destroyed. 



There is still another kind of spore called conidia, which are 

 formed on certain of the branches which grow out through the 

 breathing pores into the air. These are of a globular form and 

 much smaller than the others, and they develop into mycelium after 

 they fall off, in precisely the same manner as the zoospores after 

 they come to rest. So small are these conidia that the slightest 

 breath of wind can waft them across the field, and they may be 

 carri(!d from field to field by only a moderate breeze, so that one 

 infested potato field may be the means of infecting those for miles 

 in the direction the wind happens to be blowing. It is a noteworthy 

 fact that this parasite on the potato thrives best in a warm, damp 

 spell of weather, in fact without moisture it does not seem to thrive 



