PHYCOMY* ETES— (a) OOMYCETES 



40/ 



*%zm 



numerous small white powdery bodies, which are really comparable 

 with the sporangia of Pytkium ; these are very readily detached. 

 These sporangiophores spring from the mycelium, which permeates 

 the mesophyll of the leaf in the 

 diseased patch (Fig. 307). The 

 mycelium consists of coarse non- 

 septate, branched hyphae, which 

 traverse the intercellular spaces, 

 coming into close contact with the 

 moist walls of the cells. They are 

 also able to penetrate the softer 

 middle-lamella of the cell-walls, 

 where two cells adjoin, and this 

 brings them into still closer relation 

 to the cells, and the nourishment 

 which these can supply (Fig. 306). 

 Though the cells of the potato are a 

 not as a rule perforated, they lose 

 their vitality and collapse, probably 

 owing to a toxic influence. The 

 brown discolouration at the centre 

 of the infected spots is due to their 

 decay. 



The rapidity of the spread of the 

 disease is one of its most surprising 

 features. The fact that it habitu- 



Fig. 306. 



Piece of the tissue of the stem of a Potato- 

 plant, showing the hyphae of Phytophthora 

 penetrating the middle lamella of the cell-walls, 

 ally Spreads down the prevailing a=nucleusof a cell. Highly magnified. (After 



wind indicates 



Marshall Ward.) 



that it is due to 

 wind-borne sporangia. The sporangiophores project through the 

 stomata on the lower surface of the leaf and branch repeatedly 

 (Fig. 307). The end of each branch may swell into an inverted pear- 

 shaped sporangium, which is constricted off from its very thin stalk. 

 If growing in still air the first sporangium may be turned aside, the 

 stalk growing on sympodially, and proceeding to form a second 

 sporangium, and so on (compare Fig. 179, p. 258). Thus a suc- 

 cession of sporangia may be produced for a considerable time, each 

 readily detachable and easily borne by the wind. 



Germination takes place only in presence of moisture. The proto- 

 plasm of the sporangium divides into about ten parts, which by rupture 

 of its apex escape into the water in the form of zoospores, very like 

 those of Pythium ; they are motile for a time by means of two cilia 



