368 A TEXTBOOK OF BOTANY [Ch. VII, 5 



dermis. Then the feeding threads, or mycelia, branch every- 

 where throughout the tissues of the host, which they pene- 

 trate by aid of enzymes secreted by their tips; while the 

 walls and protoplasm and foods thus digested are absorbed 

 and used in the further growth of the mycelium. Sometimes 

 the Fungus takes no great toll and the host continues to live ; 

 but often the damage proceeds even to the death of the host. 

 It is not solely, often indeed not principally, by the robbery 

 of food that the parasite works the injury, but by poisonous 

 substances, by-products of its metabolism, released in the 

 tissues of the host. Thus parasitic Bacteria release into 

 the bodies of animals, including man, the poisonous toxins 

 which are the real cause of death from bacterial diseases 

 (page 173) ; and in analogous manner parasitic Fungi cause 

 damage and death to large areas of tissues, with resultant 

 formation of brown or black spots, which often spread until 

 the host's nutrition falls below the necessary minimum, and 

 it dies. In addition, Fungi also cause a kind of paralysis 

 of the growth-control mechanism of their hosts, resulting 

 in the formation of many kinds of monstrosities (page 199), 

 especially diseased woody growths. In some cases the Fun- 

 gus attacks parts which are not vital, such as the heart wood 

 of trees; but others attack the most essential parts, as in 

 case of the Chestnut disease, where the Fungus destroys the 

 living cambium. In any case the Fungus sooner or later 

 sends out to the surface of the host its spore-producing struc- 

 ture, from which the ripe spores are wafted by the wind, this 

 "fructification," as it is sometimes called, being often the only 

 part of the parasite visible on the outside of the host. 



Physiological disturbances include sun scalds, where the 

 sensitive tissues are injured by too much sunlight ; oedema, 

 an osmotic disease (page 234) ; chlorosis, or failure of 

 chlorophyll to develop, producing unhealthy mottled leaves ; 

 and a number of others. One might almost include death 

 from old age in this category, although plants differ much 

 from animals in this relation (page 114). 



