120 



PHYCoarrcETES. 



tufts, and form a white border round the brown parts of 

 the leaf; they are monopodially branched and produce terminal 

 sporangia (gonidia), which are easily detached. The sporangia 



on germination either pro- 

 duce a varying number of 

 zoospores, or germinate 

 directly like conidia to form 

 a mycelium capable of pro- 

 ducing new conidia. The 

 potato -disease is distin- 

 guished from Phytophthora 

 omnivora in the absence 

 of sexual reproduction by 

 oospores.' It is generally 

 assumed that the mycelium 

 hibernates in potato-tubers, 

 from which the fungus 

 recommences to spread in 

 spring. Boehm,^ however, 

 contests this, and holds the 

 hibernation of the fungus 

 to be quite unknown, and 

 that from the tubers of 

 a diseased plant, either a 

 healthy plant or none at 

 all results. 



The Phytophthora potato- 

 disease is quite distinct from 

 (a) the potato-blight or wet-rot which, according to Boehm, is the 

 result of closing up of the lenticels, with a consequent stoppage 

 of respiration ; (&) bacteriosis, which will be considered amongst 

 the bacterial diseases of plants. 



Lagerheim* has pointed out that Solanum muT-icatum 

 much cultivated in Ecuador on account of its edible fruit, has 

 been for many years subject to attack from Phytophthora 

 infestans ; the fruits sicken and rot off before ripening. The 



' This is a well-known point of controversy, for an interesting discussion of 

 ■which we would refer to "Diseases of crops" Worth. G. Smith, 1884. (Edit.) 



2 Boehm, Sitzungsher. d. Zool.-botan. Oes., Vienna, 1892. 



'■> Rivista Ecuatoriana, 1891. 



Fig. 31,— Surface of a Beech-seedling with swarm- 

 spores a, h ; the germ-tubes from these penetrate 

 between adjacent epidermal cells ; c, sporangium with 

 zoospores already germinating inside it, ^, /; £, a 

 germ-tube which has penetrated directly into an 

 epidermal cell; g, germ-tube which, after growing 

 for a time inside a cell, has again made its way out! 

 ■(After E. Hartig.) 



