384 PLANT PHYSIOLOGY 



symbiosis cannot be sharply delimited. The parasite infecting 

 the tissues of a plant in some cases depresses it; in other cases, 

 it stimulates the host plant. Frequently, if the infection does 

 not surpass certain limits, it does not appreciably affect growth 

 and development of the host plant. 



Infection mth fungi likewise considerably influences the water 

 balance of plants. Injuring the protective tissues and frequently 

 filling the stomatal openings with their hyphae and thus hinder- 

 ing their closure, the parasitic fungi usually contribute to a 

 considerable increase of transpiration. This is of little conse- 

 quence in a humid climate; but under conditions of drought, 

 the loss of the capacity of controlling stomatal movements and 

 the injury to the cuticle may be very harmful. 



The injury is still greater when the conductive tissues are 

 blocked. This occurs where the fungal hyphae fill the lumens 

 of the vessels and tracheids and hinder the ascent of water. The 

 result of such an infection, frequently caused by Fusarium and 

 other closely allied genera, is the wilting of plants under condi- 

 tions apparently most favorable for water supply. Such diseases 

 often cause great damage in fields of sunflower, cotton, etc. 

 Infectious wilts most frequently strike young seedlings, but 

 sometimes mature plants, apparently healthy, succumb to this 

 disease. 



A very widespread tyi^e of injury is the poisoning of cells and 

 tissues by toxic substances excreted by the fungal hyphae. 

 Such poisonous substances are designated by the general term 

 ''toxins." As has been shown by the recent investigations of 

 Schaffnit (1932), toxins are excreted by the fungi into the sur- 

 rounding medium. Penetrating into the conductive tissues of 

 the plant, they may cause the poisoning and wilting of the 

 tissues and organs, chiefly leaves and young stem tips to which 

 they are carried by the transpiration stream. According to 

 Schaffnit, the cause of infectious wilt, in particular the disease 

 caused by Fusarium, is intoxication of the tissues and not block- 

 ing of the conductive vessels. His investigations have proved 

 that the toxins represent nonvolatile crystalline substances, not 

 inactivated by boiling, with an alkaline reaction, and approaching 

 in their properties diamino acids formed during the hydrolysis 

 of proteins. Their action is not specific. The same toxin is 

 equally poisonous to different plants. 



