PYTHIUM. 321 



soon produce a germ-tube. It is this tube, developed direct from 

 the gonidangium, or from a swarm- spore, which penetrates through 

 the epidermis into the stem and leaves of the potato-plant, and 

 can in this way, as may be proved, infect a completely healthy 

 plant. The rapid multiplication of the parasite is provided for 

 by the formation of gonidangia and swarm-spores. 



Sexual organs have not yet been found upon Phytophthora 

 infestans, although know T n for the nearly allied Peronosporeae. 

 In these, mycelial branches in the interior of the host swell, 

 usually at their end, spherically, and form the oogonia by 

 cutting off these swellings by partition walls. By each oogonium 

 is found a mycelial branch, with its end cut off as an antheridium. 

 The greater part of the protoplasm present in the oogonium forms 

 a central globular egg-cell or oosphere, The antheridium puts 

 out a fertilising sac to the oosphere, and the resulting oospore 

 surrounds itself afterwards with a firm membrane. 



Pythium. One of the best of the Peronosporeae for study, 

 and an exceedingly likely one in which to obtain the sexual 

 organs, is Pythium de Baryanum, one of the group of parasitic 

 thread fungi which are responsible for the disease known (from 

 its results) as the "damping off" of seedlings. This disease, 

 often disastrous in its widespread results, is specially induced 

 in seedlings grown under unhealthy conditions, e.g., too closely 

 crowded, .too wet, insufficiently exposed to light and air, etc. The 

 particular fungus named above occurs almost with certainty upon 

 seedlings of common garden cress, Lepidium Sativum, when 

 thickly sown in a flower-pot about two-thirds filled with soil or 

 cocoa-nut fibre refuse, covered with a glass plate, set in a saucer 

 of water, and kept super-plentifully supplied with moisture. The 

 seeds germinate normally, but in a few days the hypocotyl will be 

 seen to bend over, owing, apparently, to a serious spindling of 

 the stem just above ground level. A few days later the lower 

 parts of the stems will be seen to be enveloped in a feltwork 

 of fungal hyphae, which, commencing in each case near the base 

 of the hypocotyl, spreads gradually over the entire plant, causing 

 its decay. Over the surface of the ground the hyphae may spread 

 to, and infect, other plants. 1 



If now a portion of a collapsed stem be examined in water 



1 See an account, illustrated, by H. Marshall Ward, in Q. J.M.S., vol. xxiii., 

 p. 487 et seq. 



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