Pethybkidge — ExferimenU tvith Phiitophthora. 21 



growu well and produced healtliy foliage ; but, on that date, suspLcious- 

 lookiug spots were seen ou the leaflets of one of the plants growu from 

 a healthy set. Microscopic examiuation showed that Phytophtliora was 

 preseut. The plant was removed immediately; hut during the next few 

 days Phytoplithora gradually appeared in isolated spots on tlie leaves of 

 seven of tlie plants from healthy, and four of the plants from the diseased, 

 sets. These affected plants were removed immediately they showed signs 

 of the disease; and by May 9th two plants only were left, one derived from 

 a healthy, and one from a diseased, set. In view of the stress wliich is so 

 frequently laid on the necessity of a high temperature and considerable 

 moisture for the occurrence of Phytoplithora, it may be regarded perhaps 

 as somewliat surprising tliat the fungus should liave developed and spread 

 in this manner in a cool greenhouse during the month of April. The 

 mean outside temperatures during the period in question were : — maximum 

 o2-B° F., minimum 35'8° P. ; and within the cool house they probably 

 ranged from 57° F. to 38° F. Altliough absolute proof is lackiug, it seems 

 practically certain that the plants whose foliage became diseased must have 

 become infected by means of " spores " from the single diseased sprout 

 sent above ground by one of tlie diseased sets. There can be no question 

 witli these plants as to infection direct from the mycelium in the sets, for tlie 

 attack began on plants derived from the healthy ones ! It seemed hardly pos- 

 sible tliat the two plants left could have escaped contamination by " spores " 

 like the rest and would remain free from disease for long ; but yet during 

 the period from May 9th to June 14th, extending therefore over more than 

 five weeks, they were allowed to remain in the cool house, and no signs of 

 Pliytophthora made any appearance on either of them. 



On the latter date after a final, most thorougli search for any incipient 

 signs of the blight, which gave an absolutely negative result, these two 

 plants — one derived from a healthy and the other from a diseased set — were 

 placed in the warm greenhouse mentioned above. Each of them was covered 

 with a large bell-jar, and tliey remained under these conditions for a period 

 of four weeks all but one day. During this time the bell-jars were occasion- 

 ally removed for short intervals to permit of the thorough examiuation of the 

 plants and to water the pots, which was but rarely necessary. »So moist was 

 the atmosphere within the bell-jars tliat intumescences were formed on the 

 leaves of both plants; and such conditions must have been ideal for the 

 development of Pliytophthora. Nevertheless no signs of it ever appeared on 

 the plants. As they grew, parts of some of the leaflets, chiefly their tips in 

 both cases, came into contact at one or two points with the tops and sides of 

 the bell-jars. Where this occurred the tissues decayed into a slimy material 



p2 



