220 PHYTOPHTHORA SP. ON HEVEA BRASILIENSIS 



This formation of such a stroma-like body has not been, as yet, observed 

 in any of the PhytopJithoras known. The only other genus of the Perono- 

 sporacecB in which something akin to it is formed is Cyslopus and even here the 

 base of the sorus consists merely of the swollen pedicels of the spore-chains 

 and no formation of distinct basal cells interposed between the mycelium and 

 the spore stalks occurs. De Bary considers that some characters found in 

 Pythium suggest a common ancestry for Cystopus and PhytofUhom and 

 therefore the finding of this sorus-like structure in a Phytophthora is of 

 interest as recalling the sori of the Cystopiis. In Cystopus, however, the 

 hyphffi beneath the epidermis bear directly a tuft of broad basidia from 

 each of which a single chain of sporangia arises while in the Phytophthora 

 under study the basidium or apical cell of the stromatoid body bears not 

 alone one but as many as three sporangiophores. 



The sporangiophores vary much in length ; at times they are extremely 

 small, the sporangia being almost sessile on the epidermis. Sometimes the 

 spore stalk instead of bearing a sporangium directly after it has emerged from 

 the epidermis continues its vegetative growth, and sends out branches which 

 ultimately bear sporangia. The sporangiophores are simple ; only occasionally 

 branched. Sporangia are ovate or citron-shaped and terminal ; at times 

 lateral. The number of zoospores in a sporangium varies with the size of the 

 sporangium. They are bean-shaped with a pointed anterior and have two 

 cilia of unequal size. After swimming for some time the zoospores come to 

 rest, lose their cilia, round ofE and germinate by sending out one to three 

 germ-tubes which either continue their vegetative growth or when quite small 

 swell at the tips to form secondary sporangia which when mature liberate 

 one to three zoospores (Fig. 2, p. 218). Resting conidia are borne in the felt 

 of mycelium on the surface of the fruit and on the tapping cut. They are 

 honey-coloured, yellowish or at times hyaline, generally round, and they 

 are either terminal, lateral, or intercalar. 



Inoculations on hevea. 

 In order to verify that this Phytophthora was the cause of the " Black 

 Thread " disease, inoculation experiments with inire cultures obtained from 

 diseased fruits have been carried out. The inoculations were at first done by 

 putting a bit of the mycelium on a newly tapped surface ; the inoculated 

 areas were kept covered from 24 to 48 hours with cotton soaked in sterilized 

 water. In four days the points inoculated sho\\ ed signs of successful infection : 

 they had turned either black or had vertical black lines, and had become 

 slightly depressed. Within a week they showed typical symptoms of " Black 



