182 PHYTOPHTHORA PARASITICA. 



membranous portion gives way. possibly under pressure and the 

 fungus continues its course inwards or outwards as the case may be 

 (Plate II, Fig. 3). 



Internal sporangia have often been observed in the tissues of 

 leaves, stems and fruits. They have not been found either to germi- 

 nate conidially or to produce zoospores in leaves and stems, but in a 

 few cases they have been observed to germinate conidially in the 

 soft tissues of the pericarp of the fruit (Plate V, Fig. 15). Of 

 what use they are to the parasite it is difficult to say. They are 

 known to be formed in the leaf within twenty- four hours after inocula- 

 tion. Similar bodies have been found in the leaves of potatoes 

 attacked with Ph. infestans 1 . 



I have failed up to now to find typical oospores in nature. It 

 is possible that they are formed, as in some artificial media they are 

 readily produced, but they may be very few and so escape detection. 

 Very rarely on mashing old leaves boiled in caustic potash resting 

 bodies have been found which might be parthogenetic oospores. 



Infection Experiments on the Castor Oil Plant. 



From the many series of inoculation experiments carried out 

 in the laboratory it has been found that leaves young and old most 

 readily fall victims to the attack of the parasite. Before it was 

 taken in pure culture inoculations were made in either of the two 

 following ways : — (1) A bit from a diseased portion of a leaf from the 

 field was placed with a drop of water on either of the surfaces of a 

 leaf of a healthy plant which was then covered by a bell jar to keep 

 it in a moist atmosphere. After about six hours the diseased piece 

 used for inoculation was removed. (2) A diseased leaf from the 

 field was kept for twenty-four hours slightly immersed in water in a 

 covered Petri dish. The fungus grew out on the surface of the water 

 in the Petri disband produced sporangia in abundance ; from the 

 surface not in contact with water an aerial growth was produced. 

 Either zoospores suspended in water or a little of the mycelium 



' Jones, L. R., Giddings, N. J., and Lutman, 13. P. Investigations of the Potato Fungus 

 Phytophthora infestans. U.S. Dept. Agric., Bureau of Plant Industry, Bull. No. 245, 1912, p. 2is. 



