458 Prof. H. Marshall Ward. On Pure Cultures 



Since each tube is linked to its neighbour with clean flexible tubing, 

 and the air bubbles through the liquid in the reservoir (C), and has to 

 pass the cotton-wool plug (B) before reaching the leaves (F) in the air 

 above, there can be no question of infection from outside, and the 

 results also show that infection only occurs exactly where I place the .yores 

 on the leaf in each case. 



The spores employed were carefully tested as regards their germi- 

 nating power, and, as the table shows, the results in the closed tubes 

 fully bear out previous experience. In the aspirated tubes, however, 

 the second pair of seedlings of B. mollis(No. 712) gave negative results, 

 inasmuch as only flecks, and not pustules bearing spores, were de- 

 veloped. In the closed tubes, however see below the positive 

 results, especially on B. velutinus and B. secalinus, were excellent, and 

 subsequent examination showed that the spores germinated well, and 

 were capable of infecting other seedlings. 



In order to test further the behaviour in mineral solutions, I pre- 

 pared, as Table III shows, several series in closed tubes, Nos. 713, 

 which served as a parallel series to Nos. 711 and 712, but without 

 aeration. 



In No. 713 the sterile seedlings were raised antiseptically as before, 

 but the roots merely penetrated cotton-wool saturated with Knop's 

 solution, and held by the constriction over the bulb filled with the- 

 same, no air being drawn through. 



The growth was excellent, and the results very conclusive, as Table 

 III shows. 



The seedlings were allowed two days at 22 20 C. in the laboratory 

 and then put out side by side with 711 and 712 in full sun during the 

 middle of the day, and after two days' further growth were infected. 



By the 10th day the thin leaf was well developed, and the first 

 pustule was seen on B. mollis and B. secalinus on the eighth day after 

 infection. The Table III shows the course of events in detail. 



The growth of pustules was excellent on B. velutinux and B. seca- 

 liniis especially. 



This experiment is interesting, not only as showing that plants can 

 be grown and infected successfully in these closed water-cultures, but 

 especially as showing the contrast between the aerated and non-aerated 

 tubes, for, since the infected seedlings were selected in each case from the 

 same Petri-dish cultures, we must assume that the difference in rate of 

 development was due to the difference of ventilation, and perhaps con- 

 clude that this interferes with the success of the parasite, as measured 

 by the somewhat longer incubation period. It is remarkable how 

 dwarfed the continuously aerated plants are, compared with those in 

 closed tubes, owing to the elongation of the leaves of the latter. 



It is clear, therefore, that pure cultures of Uredo-spores can be ob- 

 tained by this method, and it is equally clear that we can also obtain 



