CHAPTER IV 

 ANCYLISTALES 



The members of this order are all endobiotic parasites, the 

 thallus in most eases being confined to a single host cell. The 

 parasite gains entrance to the host by means of a delicate infection 

 tube, which on reaching the interior rounds up at the tip to form a 

 small spherical bladder. This bladder by further growth 

 develops into the thallus. The young thallus is a rather short, 

 unicellular, more or less cylindrical tube. In some species it is 

 somewhat branched or lobed. At maturity, it is usually divided 

 by transverse septa into a number of cells. The component cells 

 of the thallus may function variously, some as sporangia, some 

 as gametangia, and some vegetatively. In the last case the 

 cell germinates directly by a slender germ tube, which may 

 become much elongated and serve as an infection thread by 

 means of which the fungus enters another host cell. The cell 

 of the thallus in this case is perhaps to be regarded properly as 

 a sporangium which has assumed the method of germination 

 characteristic of this structure in the higher Oomycetes. This 

 type of germination is relatively rare in the order, the sporangia 

 in most species forming swarmspores. In some cases all the 

 cells of a single thallus function alike. Sporangia and sexual 

 cells are developed in the same or different thalli, a given species 

 being usually either homothalHc or heterothallic. 



The sexual process in the group has not yet been adequately 

 studied. The cells of the thallus which function as male and 

 female gametangia respectively show a relatively slight degree 

 of morphological differentiation as compared with those of the 

 Oomycetes. The female is typically larger and more character- 

 istically barrel-snaped than the male which is commonly cyUn- 

 drical. Usually copulation of the two cells is accomplished by 

 means of a more or less elongate conjugation tube. The terms 

 oogonium and antheridium may be used only tentatively since 

 oosphere formation has not been convincingly demonstrated. 

 In Ancijlistes the oosphere seems to be clearly absent. If present 

 in other members of the group it is at best imperfectly differ- 



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