128 TECHNICAL BULLETIN 4. 



Progressive Infection of New Leaves. 

 It is a common belief, supported by statements in the literature of the 

 disease, that when a seedUng once becomes infected it never recovers. 

 Such, however, is not the case. The waiter has watched the development 

 of many seedlings which had infected cotyledons, but which developed 

 into healthy onions. On the other hand, he has not seen an onion, in which 

 the first leaf was affected, which produced a healthy bulb. Usually each 

 successive leaf will show smut sori, and they are not always in any appar- 

 ent relation to the sori on older leaves. As previously stated, all infec- 

 tions come through the cotyledon, but the fate of the plant depends on 

 the point in the cotyledon at which infection takes place. If it occurs 

 only high up toward the knee, or above it, there is a pretty good chance 

 that the host tissue will have become mature or dead and no longer suit- 

 able for spread of the mycelium before the latter has reached the gromng 

 zone, and the bulb will develop normall3^ But if infection occurs at or 

 very near the root joint, the mycelium quickly penetrates to the gro^^dng 

 zone from which all future leaves arise. This meristematic tissue furnishes 

 the ideal condition for continuous vegetation of the pathogene, and as 

 each new leaf pushes out from this restricted stationary zone it contains 

 filaments from which the new sori of the successive leaves develop. When 

 the parasite is once established in this growing point, the host seems never 

 to be able to shake off its grip, and is doomed. It is not quite so clear why 

 the mycelium does not enter the tissues of the developing roots in the 

 same wa}', but the writer has never been able to find it in these organs, 



VI. Sporogenesis. 



The approach of spore formation is first indicated by massing of the 

 mycelium between the cells. Up to this time only long straight slender 

 hyphse are found spreading singlj^, or at most not more than two or three 

 together, between the cells. The period during which the pathogene ap- 

 pears to be spreading as widely and rapidly as possible between the cells 

 has just been described as the incubation stage. The distributive hyphse 

 now begin to branch profusely, and the branches are not straight and 

 parallel to the main hyphse, but become twisted and interwoven into 

 dense tangles which push the cells apart and increase the area of inter- 

 cellular spaces within which the spores are to be formed. The hyphse 

 now become highly vacuolated, and the protoplasm between the colorless 

 vacuoles stains densely blue with the triple stain, while the old cells from 

 which the protoplasm has passed take the orange stain. The beaded 

 appearance of the alternating vacuoles and densely staining cytoplasm is 

 the surest indication of approaching sporogenesis. 



These spore nests or sori always occur between the cells of the mesophyll 

 anywhere between the epidermis and the bundles, but have not been found 

 inside the bundles. They are extended in the direction of the length of 

 the leaf or cotyledon. 



