110 POTATO-FPNGUS. 



part live in water ; some grow in livinp; ])lants. Their mycelium and 

 the Tiiode of formation of their zoospores essentially aj^ree with 

 what is observed in Peronospora, but the I'orm of the zoosporanpia is 

 often, though not always, different. The principal difference of the 

 two families consists in the mode of formation of the oospores, for in 

 SaproUgniece the entire protoplasm of the oogonium goes to form 

 the oospores — formini; either one by SL'parating itself entiri'ly from th<' 

 oogonium and surrounding itself with the endospore and epis])ore, or 

 forming several by separating into a number of parts, each of whicli 

 possesses the structure of an oospore. The shape of all these organs 

 is always very like that of the same organs in Ferononporem, and 

 the method of germination in the oospores is also similar ; but 

 the oospores themselves are usually smaller and more slender. 

 Should, therefore, Phytophthora come to be classed with tlie 

 SiiproJpgniecc, the mode of formation of the oospores would be expected 

 to be similar to what is known to take place among its allies, just as 

 when it is placed, as it usually is, with Ihc Peronoiporea;, we ascribe to 

 it the same method of reproduction as we find in that family. 



2. The phenomena connected with the growth of the Ptroiosporea 

 are tolerably uniform ; all are typical parasites in living plants con- 

 taining chlorophyll. Their complete develo])ment is dependent on 

 their finding the living organism, with its chemical and i)hysical pro- 

 pei'ties, which will afford it a suitable host ; and most species are so 

 restricted in this respect that they can only grow in certain species or 

 groups of species of plants and not in others, a condition of things 

 which holds good for parasites generally. But this does not exclude 

 the possibility of bringing a parasite more or less forward in its de- 

 velopment by means of artificial nutriment. 



That the species of the Pef07io.<!2)orere, as parasites, do more or less 

 interfere with the life of their host and produoe disease in it must be 

 obvious ; and the circumstances under which this occurs aretoogenerally 

 known to require detailed explanation here. 



The mycelium of the Peronosporea grows in the living tissue of 

 its host, and with the greatest vigour when the foliage and flowers are 

 being rapidly produced in the summer ; this to some extent determines 

 the principal season of the growth of these Fungi. At the same time it 

 should be remembered that the mycelium of particular kinds may, in 

 the case of perennial plants, also continue to live in and along witli 

 the pcirennial parts, and annually to spread from them to the 

 parts developed in summer, and so extend itself along with these, as, 

 for instance, Peronospora Ficarm in Ranunculus Ficaria, P. Rumli'ls 

 in the Sorrel, Rumex acetosa. 



The tubes of the mycelium gro v chiefly between the cells of the 

 tissues of the host-plant, not only touching the cells, but in most 

 species also pushing their small branches, whicli I have before calle<l 

 suckers, into the interior of the cells. 



In certain species, which do not concern us here, the conidiophores 

 are formed only on very special regions of the host ; in most species 

 they may grow wherever there is mycelium. They are produced when 

 the mycelium passes out from the tissues of the plant into the air — 

 that is, with rare and casual exceptions, upon tlie surface of the plant, 

 particularly the foliage. In Cystopus they form thick layers under the 



