BOTANICAL GAZETTE. 307 



genera in question. In Phytophthora and Peronospora the 

 mycelium in the interior of the leaves makes its way to the sur- 

 face, in most cases passing through the breathing pores. That 

 part of the mycelium which passes into the air and bears the 

 conidia is called the conidiophore. In Phytophthora it is a sim- 

 ple or branching filament, at whose tip or tips is formed an ovate 

 cell, the conidium. Before the conidium falls from its attach- 

 ment the conidiophore, at its base, swells on one side and then 

 grows upward, pushing the conidium to one side so that it ap- 

 pears to have been formed by a lateral outgrowth. The elon- 

 gated conidiophore then bears a new conidium at its tip, which 

 is in turn pushed to one side by the growth of the conidiophore, 

 as before. The genus Phytophthora may then be recognized 

 either by having one or more conidia borne laterally on the con- 

 idiophore, or, as is more frequently the case, by the swellings at 

 intervals in the upper part of the conidiophore, indicating where 

 conidia have fallen off. 



In the genus Peronospora the conidiophore is a branching 

 or rarely a simple filament which bears the conidia singly at the 

 tips which do not afterwards elongate, as in Phytophthora. In a 

 few species where the conidiophore is rather robust, instead of 

 bearing a single, conidium, the tip bears a number of short pro- 

 cesses, or teeth, on each one of which is borne a conidium, as in 

 P. entospora and P. gangliformis. 



In Cystopus the mycelium, instead of passing through the 

 stomata into the air, collects in spots under the epidermis, which 

 is then ruptured and turned back, and the filaments which cor- 

 respond to the conidiophores in the other genera are very short 

 and stout, and densely packed together and grow into simple 

 chains of conidia by transvere cell division. Hence, while Phy- 

 tophthora' and Peronospora: appear to the naked eye like fine 

 frost like patches, especially on the under surface of the leaves, 

 species of Cystopus produce dead white spots not limited to the 

 under surface of the leaves. 



The species of Phytophthora and Cystopus are few in number 

 and it is not necessary to subdivide these genera, but as there is 

 a considerable number of species of Peronospora it is usual to 

 subdivide the genus. The subdivisions made by DeBary depend 

 upon the mode of germination of the conidia; in the first divis- 

 ion he places the species whose conidia give off a number of 

 zoospores in germination ; in the second, including only two 

 species, the whole contents of the conidia are discharged in a 

 mass which is not however motile, but which at once pushes 



