328 FRANCIS O. HOLMES. 



cance in their relation to the biology of the flagellate than had 

 at first been realized. 



It had been noticed that infections did not spread from plant 

 to plant in groups connected by a common axis. Such plants 

 arose from separate buds, the latex systems of which were 

 independent of each other. In the axis itself no ducts occur, so 

 that there is no chance for wandering through such a connection. 



Late in the autumn of 1924 two plants which had been under 

 observation for months were dug up to determine their exact 

 relation to each other. One of these had been consistently 

 negative for flagellates all summer. The other had been as 

 consistently heavily infected in every part above the ground, 

 with the exception of the seeds which are always free from 

 invasion. The two plants were separated from each other by no 

 more than six inches of axis, from which common source they 

 had both sprung as buds. The absence of latex ducts in the 

 axis and the confinement of the flagellates to the latex of the 

 infected plant made it impossible that the nearby negative plant 

 should be invaded except from some outside source of the 

 organisms. 



The practical confinement of the insect, Oncopeltus fasciatus 

 (Dall.), suspected of being the insect host of the flagellate, to 

 the blossoms and pods of the milkweed plant in feeding is also 

 made significant by histological studies of the latex system, 

 which in these two parts becomes much more prominent than it 

 is in the stems or leaves. The soft tissues under the outer green 

 coverings of the pods contain numerous branches of the latex 

 system, and in the area between the pedicels and the bases of 

 the petals of the flowers the latex ducts are exceedingly close 

 together and voluminous. This offers a favorable feeding loca- 

 tion to the insect, and, by reason of the crowded flagellates here 

 in infected plants and the softness of the tissues, opportunity is 

 offered for the infection of insects from the plant and for the 

 infection of previously uninfected plants by the insects. 



THE FLAGELLATES OF Oncopeltus fasciatus (Dall.). 



It is not yet known with certainty whether Oncopeltus fasciatus 

 (Dall.) is the insect which carries the milkweed flagellate from 



