270 RESEARCH IN PROTOZOOLOGY 



Such plants recover, however, and appear healthy by the time 

 flagellates become numerous in their latex. Similar plants infected 

 equally heavily by a single insect may show no injury at all. 



Seasonal changes in the host plant may be confused with the 

 effect of flagellates if careful observations are not made. In 

 Asclepias syriaca under the conditions in the United States 

 (Holmes, 1925a) the presence of the non-pathogenic species Her- 

 petomonas elmassiani has been found to bear a relation to yellow- 

 ing and ripening changes in the host plant after seed production 

 in the fall. This does not indicate that the organisms are the 

 cause of the yellowing. Plants which fail to blossom appear green 

 and vegetative, and are not found to contain flagellates. Yet such 

 immature plants may be infected by confining the insects on them, 

 and when so infected remain green and vegetative as before. Ex- 

 periments and observations have shown that the flagellates are 

 absent from the green plants because the insect host prefers to 

 feed on plants which have blossoms and fruit. The yellowing and 

 ripening changes in these older plants, however, are independent 

 of the presence of the flagellates and depend wholly on the normal 

 processes accompanying seed formation in the host plant. 



By means of such observations it will be necessary to determine 

 for each species of latex flagellate discovered in the future its 

 relation to the health of its host plant. Some of the flagellates of 

 Enrphorbias already studied need more careful work of this sort. 

 Climate, seasonal changes and other environmental factors may be 

 the causes of the differences of opinion, but they must be care- 

 fully studied before being considered the responsible factors. 



Effect of ftagellatcs on the health of tJie insect hosts. Thus far 

 no observations have shown the exact effect of the presence of 

 latex flagellates in their insect hosts upon the health of the insects. 

 But it is possible that injury may occur in some cases. One diffi- 

 culty in conducting studies of incubation periods in the insect 

 host is the frequent death of insects before they become infective. 

 These deaths are numerous at the time when the insects would be 

 expected to become infective. Careful examination should be 

 made to see whether there may be intracellular stages of the flagel- 

 lates at this time, and corresponding damage to tissues preceding 

 entrance into the salivary glands. Little is known about this, but 

 methods are available for a careful study of the progress of the 

 infection in the insect host. Since the insects can be reared in some 



