122 TECHNICAL BULLETIN 4. 



by the length of time required for the seedlings to reach a certain stage, 

 it is well to inquire how the rate of growth in the greenhouse compares 

 with that in the field. During the spring of 1920, when the spring was 

 late and cold, onions in the field did not come up for over two weeks in 

 most cases, but growers have frequently told the writer that they have 

 had fields which came up within eight days. Apparently weather and soil 

 conditions may materially affect the length of this period. Depth of 

 planting might also influence slightly the length of the period and also the 

 chances of infection. The experiment reported above, however, gives us no 

 information as to the date when infection begins, but only indicates that 

 it ends -with about the seventeenth day. 



In order to determine the stage at which the earliest infection starts, — 

 and at the same time to work out other points in the early life history, — ■ 

 another bed of onions was started in the greenhouse with soil knowai to 

 give 100 per cent of smut infection. Beginning with the third day, a cer- 

 tain number of plants was dug up each day, fixed in Flemming's weaker 

 solution, run up into paraffin, sectioned, mounted serially and stained 

 with triple stain. No mycelium was found in the tissues of those which 

 were fixed on the third and fourth days. The first infection was found 

 in a plant which was dug up on the fifth day after planting, and was 

 apparently a very young infection because it had at no point penetrated 

 more than to the fifth layer of cells below the epidermis, and at its furthest 

 point was not more than 150/; from the point of infection. Fifteen other 

 plants dug at the same time were carefully searched under high power 

 through every section of 92 slides, but no other trace of mycelium was 

 found. It is probable, therefore, that only rarely, if ever, has the my- 

 celium entered the tissues of the plant on the fifth day after planting 

 (second day after germination has started). Since cultural experiments 

 with the smut fungus have shown it to be of very slow growth, at least in 

 the saprophytic condition, it seems hardly possible that it could have 

 succeeded in entering the tissues before the second day after germination 

 of the seed starts. 



It may be concluded from everything which has been learned up to the 

 present in regard to the period of susceptibility that infection may take 

 place at any time between about the second day after the seed starts to germinate 

 until the seedling is in the first leaf (a period of about twelve days in the 

 greenhouse) . 



Point of Infection. 

 In the studj^ of the plants fixed and stained as mentioned above, many 

 very young infections were found where it was possible to determine the 

 point of entrance for the mycelium. Infections were found at the knee 

 above, at the root joint below, and at various points between, also at least 

 one through the interior wall of the cotjdedonary cavity. The conclusion 

 is, therefore, that all points of the epidermis at least between the root 

 joint and the knee are susceptible to penetration by the smut tubes. In- 

 fection was never found taking place in the roots proper or between the 



