ii DEVELOPMENT AND ENVIRONMENT 19 



culture, they are visibly infected in three hours and 

 completely destroyed in a few days. Repetition of the 

 experiment at 33 C. produces slight superficial brown 

 scars, and nothing more, while at 37 C. the seedlings 

 remain without injury. Moreover, if an infection has 

 been established at 20 C., and is then transferred to a 

 temperature of 37 G. for an hour or two, or for several 

 hours to 33 C., the infection is arrested. The hyphae 

 cease to advance, and the host proceeds to delimit the 

 damaged portion by means of a cork cambium. 2 



The effect of high temperature can be modified in one 

 way. If the tissue to be infected is partially immersed in 

 water and kept at 33 C., the fungus will' destroy all the 

 cells on the water-line, though merely scarring the epi- 

 dermis above, as before, and leaving the immersed portions 

 untouched, on account of deficient aeration. 



The results of infections in the field depend on the 

 age of the plant. The brown skin of seedling roots is 

 commonly regarded as normal to the cotton plant, whereas 

 it is actually due to abortive attacks by this fungus and 

 possibly by a Rhizopus. Very late sown seedlings have 

 pure white roots. The seed is doomed if the attack takes 

 place as soon as the tip of the radicle emerges from the 

 seed coat ; its growing point is destroyed, while the fungus 

 enters the seed-coats and rots the cotyledons ; such seed 

 is commonly said to have been " killed by the cold." 

 When the attack is made on an established root or 

 hypocotyl, the infection has to be sufficiently extensive to 

 reach the phloem tissues ; such destruction of the phloem 

 limits the supply of synthcsised food from the leaves to 

 the root, and may check root-growth ; this check in its turn 

 limits aerial growth, and so produces stunted plants, which 

 flower late and make the crop irregular. If much phloem 

 tissue is destroyed, the seedling may die through indirect 

 water shortage during the day, but though seedlings which 

 have wilted from this cause arc common in the field, the 



c 2 



