120 



PHYCOMYCETES. 



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 Phytopliihora 

 omnivora in the absence 

 of sexual reproduction by 

 oospores. J It is generally 

 assumed that the mycelium 

 hibernates in potato-tubers, 

 from which the fungus 

 recommences to spread in 

 spring. Boehm,' 2 however, 

 contests this, and holds the 

 hibernation of the fungus 

 to be quite unknown, and 

 that from the tubers of 



FIG. 31. Surface of a Beech-seedling with swarm- a diseased plant, either a 

 spores a, ') ; the germ-tubes from these penetrate 



between adjacent epidermal cells ; c, sporangium with healthy plant 01' 110116 at 

 zoospores already germinating inside it, d, /; c, a 

 germ-tube which has penetrated directly into an all results, 

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



for a time inside a cell, has again made its way out. The PhlltOljllthoTa POtatO- 



( After R. Hartig.) J f 



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 ; (I) bacteriosis, which will be considered amongst 

 the bacterial diseases of plants. 



Lagerheim 3 has pointed out that Solatium muricatum 

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

 been for many years subject to attack from Phytophthora 

 the fruits sicken and rot off before ripening. The 



1 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.) 



-Boehm, Sitzunysber. d. Zool.-botan, Ges., Vienna, 1892. 

 3 Sivista Ecualoriana, 1891. 



