AMERICAN JOURNAL OF BOTANY [Vol. 9, 



when, shortly after inoculation, the host is placed under conditions where 

 it cannot manufacture carbohydrates, as by keeping it in the dark or in 

 light from which the red-orange end of the spectrum is filtered off or in air 

 deprived of carbon dioxide, the development of the rust is inhibited. 



Fromme (1913, p. 516) found that placing oat plants recently inoculated 

 with Puccinia coronifera in the dark for a period of several days increased 

 the length of the incubation peiiod of the rust by a corresponding interval. 

 Fromme interprets the observation to indicate that the fungus is dependent 

 for its nutrition on some intermediate product of photosynthesis. 



Mains (1917, p. 191) confirms Fromme's observation that the develop- 

 ment of the crown rust of oats is retarded in the absence of light, and adds 

 that if the infected plant is left in the dark too long (which would greatly 

 impair the vitality and vigor of the host tissue) the rust is killed. Mains 

 also independently repeated Ward's observation that growing the host 

 plants in an atmosphere free from carbon dioxide inhibits the development 

 of the rust. Similar experiments with Puccinia Sorglii on seedling plants of 

 Zea Mays, however, failed to arrest the development of the rust. Further 

 experiments showed that if the host leaf is supplied with carbohydrates, 

 either from the reserve stores of the endosperm or by being floated under 

 aseptic conditions on a sugar solution, then the rust develops successfully, 

 even if, because of the absence of light or of carbon dioxide, the host tissue 

 cannot manufacture its own carbohydrates. Mains is therefore prompted 

 to qualify Fromme's inference that the rust is dependent upon intermediate 

 products of photosynthesis into the statement that the rust is dependent 

 upon transitory carbohydrates. 



It does not necessarily follow from an observation that the development 

 of the rust is inhibited upon a plant starved of an essential nutrient 

 whether carbon, or nitrogen, or potassium, etc. that the rust fungus is 

 dependent for its nutrition upon compounds of that substance. As long 

 as the host plant is at all alive, or, even if it is dead, before disintegration 

 of its substance has set in, it contains carbon, nitrogen, etc., compounds, 

 and we cannot say that the rust could not develop because of the absence of 

 such compounds. Such observations are best interpreted on the basis of 

 the physiological condition of the host when it is starved of an essential 

 nutrient substance. We can not say that a host plant starved of an essential 

 nutrient is a host plant deficient in that particular class of substances; but 

 we can say that a host plant starved of an essential nutrient is a host plant 

 that is not assimilating, that is not growing, a plant in which anabolic proc- 

 esses are at a standstill and katabolic processes predominate. And we 

 are justified in inferring from the observed behavior of rust fungi on host 

 plants starved of essential nutrients that a plant which is not assimilating, 

 which is not growing, in which anabolic processes are at a standstill and 

 katabolic processes predominate*, does not make a congenial host for the 

 rust fungus. The suggestion that the rust is dependent for nutrition upon 



