1902.] 



Starvation on the Parasitism of Puccinia. 



141 



one, and the whole were now placed in a sheltered situation in the 

 open, the bell-jars being raised on large blocks so that their rims just 

 covered the rims of the beakers ; thus permitting a free circulation of 

 air, but protecting the whole from beating winds or rain. An awning 

 was also provided, and carefully adjusted during hot days as necessary. 

 Growth proceeded fairly rapidly, though of course some retardation was 

 inevitable owing to the impervious nature of the beakers as con- 

 trasted with pots having porous sides, and by July 18 each beaker 

 had from two to seven satisfactory seedlings. 



On July 23, ten of the beakers had from five to seven excellent 

 seedlings each : three had but four each, and one had only two suc- 

 cessful seedlings. So far there was but little difference observable 

 between the different seedlings, each of which was unfolding its third 

 leaf. Nevertheless, there was evidence that the stores of food- 

 materials in the indosperm were now being exhausted, and that the 

 root-system was beginning to feel the effects of the differences in 

 mineral supplies. Consequently, I fished out the remains of dead 

 or un-germinated grains, and proceeded to infect certain of the seed- 

 lings in each beaker with Uredo-spores of Puccinia dispersa obtained 

 from Bromus sguarrosus. This species was selected because I had an 

 abundant stock of vigorous spores at hand, and had already satisfied 

 myself that they readily infect Bromus secalinus. 



The infection was effected by placing equal doses of the fresh spores 

 on each leaf chosen • the bell-jars were then lowered in order to keep 

 the plants in a moist atmosphere for 21 hours, and then raised again 

 on blocks for another 24 hours, after which the bell-jars were removed 

 each morning, and only replaced at night or during rain. 



The results of infection were already evident on one or two plants 

 on July 31 and August 1, increasingly so on August 3, and by 

 August 5 the pustules were as prominent as they ever became 

 during the period of the experiment — i.e., up to August 29 — except in 

 so far as some of the larger ones ran together or produced more 

 spores. 



The results are summarised in Table I, but it should be noted that 

 it has been impossible to compress into the table the details of the 

 observations from day to day as to the stature, robustness, colour, 

 number and breadth of leaves and so on, or as to the rate of develop- 

 ment of the plants and of the flecks and pustules of the parasite. 



In order that the reader may gather an impression of the appear- 

 ance of these seedlings during the progress of the experiment, how- 

 ever, I append photographs* of one set of the beakers in fig. 1, and of 

 a representative series of the plants themselves, carefully extracted 



* I have to thank my son, F. K. Ward, for the preparation of these photo- 

 graphs. 



