324 FRANCIS O. HOLMES. 



plants, the entire freedom from penetration of tissues aside from 

 the latex systems, and the sufficient food supply presented to 

 the flagellates by the milky vacuole fluid itself in the latex cells. 



The fact is often overlooked that plant flagellates of the type 

 of Herpetomonas davidi (Lafont) and Herpetomonas elmassiani 

 (Migone) have been found only in such plants as have abundant 

 latex. Those engaged in research in this field are of course 

 acquainted with this restriction of the range of the organisms, 

 and take it into account in most of their work, but do not always 

 seem to have it very definitely in mind. Others whose interest 

 in plant flagellates arises from some other, less immediate, source 

 are often entirely unmindful of the situation until it is brought 

 to their attention. 



To the present time no acceptable species of flagellates of the 

 genera Herpetomonas, Leishmania, Crithidia or Trypanosoma have 

 been found in plants other than those prpyided with a milky 

 juice or latex. 



Perhaps the situation would be better understood if it were 

 commonly known that latex does not occur extracellularly in 

 plants, but intracellularly. Thus the flagellates which are trans- 

 ferred by their insect hosts to the latex cells of plants are not to 

 be found thereafter at random in the plant tissues, but are 

 strictly intracellular (not intracytoplasmic) parasites. 



A description of the cells containing the latex will make clear 

 the relation of any latex-inhabiting organisms to the host plants. 



The latex ducts of the plants with which protozoological 

 studies have been most concerned are those known as simple 

 ducts, because they do not fuse with each other in the course of 

 their wanderings. Among the Asclepiadaceae, Euphorbiaceae, 

 Apocynacese and Urticaceae the cells destined to become the latex 

 ducts of the mature plants are already distinguishable in the 

 embryo. Their nuclei divide again and again, and the cells 

 elongate tremendously and branch repeatedly, but no cross walls 

 are formed, nor do the ducts fuse with one another, so that 

 eventually the few original cells penetrate every part of the 

 plant, and still remain distinct and separate from each other. 

 The thin cellulose walls are lined with a layer of cytoplasm 

 containing numerous nuclei. In the vacuole is collected the 

 milky, usually white latex. 



