Apr., 1922] RAINES VEGETATIVE VIGOR OF THE HOST 187 



is the strong suggestion, therefore, in Zavitz's data that the increase noted 

 in the amount of rust present on oat plants grown at progressively greater 

 distances apart is correlated with the increased luxuriance of growth of the 

 host plants. 



Water- Culture and Sand- Culture Studies . 



Ward (19020) details two experiments on the susceptibility to rust in- 

 fection of host plants which had been starved of essential nutrients. In 

 the first experiment, 54 young seedlings of Bromus secalinus were used. 

 The plants were grown in sand in 14 glass beakers, four to seven plants to 

 a beaker, and watered with solutions -of varying nutritive value. The 

 plants in one beaker received only distilled water. The plants in another 

 beaker received a cold-water extract of fresh horse dung, as a solution of 

 high nutritive value. In a third beaker the plants received a full mineral 

 nutritive solution (described as a "normal nutritive mineral solution con- 

 taining nitrates, phosphates, and sulphates of potassium, calcium, and 

 magnesium"). The remaining eleven beakers received an incomplete 

 nutrient solution, the elements omitted being respectively K; N; Mg; Ca; 

 P; Fe; N and Fe; Mg and Fe; Ca and Fe; P and Fe. Inoculation was 

 effected by applying uredospores of Puccinia dispersa to the leaves by means 

 of a swab of cotton; at the time of inoculation the seedlings were 16 days 

 old, counting from the time of sowing. 



Ward records detailed observations on the stature, robustness, color, 

 and number of leaves of the seedlings in each beaker; on the time of appear- 

 ance, number, and size of the pustules developed on them, and on the 

 relative number of spores produced. Comparing the twelve seedlings 

 which showed the most vigorous growth (6 which received the extract of 

 horse dung, 3 the full nutrient solution, and 3 the full nutrient solution 

 minus Fe, the plants averaging 20 cm. in height) with the ten poorest plants 

 (4 receiving distilled water, 3 the full nutrient solution minus N, and 3 the 

 nutrient solution minus N and Fe, the plants averaging II cm. in height), 

 the observations recorded by Ward indicate that in the plants suffering 

 from malnutrition (i) the incubation period of the rust was lengthened by 

 one and two days; (2) the rust pustules were much smaller and produced 

 fewer spores. In other words, a starved host meant a starved parasite. 

 There is also the suggestion in the data that the starved seedlings showed 

 a lower incidence of infection; but the small number of variables worked 

 with, together with the large irregularity in dosage inherent in the method of 

 inoculation used, compel reserve in making this deduction. 



A second experiment with 64 seedlings, duplicating the first, gave 

 similar results. Regarding the spores produced on well nourished and on 

 starved seedlings, Ward states that microscopic examination revealed no 

 differences. Spores from starved seedlings could produce infection on 

 other seedlings, similarly starved. 



