46 AQUATIC PHYCOMYCETES 



growth the young thallus may assume a spherical or ellipsoidal shape 

 and in parasitic forms be carried away, perhaps by cytoplasmic move- 

 ments of the host, from the open tip of the penetration tube. This tube, 

 as well as its attendant epibiotic cyst, usually disintegrates and takes 

 no further part in the developmental cycle. Inside, the young thallus 

 absorbs materials over its whole surface and increases steadily in size. 

 No specialized vegetative parts are formed, and at maturity the body, 

 which is completely within the substratum (endobiotic), becomes con- 

 verted as a whole into a reproductive structure (holocarpic). The zoo- 

 spores in the sporangia of endobiotic types are conveyed to the outside 

 by a more or less well developed discharge tube, the tip of which is 

 extramatrical. Such a type of development is characteristic of the Ol- 

 pidiaceae. It is elaborated in the Achlyogetonaceae, in which family 

 the thallus is cylindrical, and at maturity becomes segmented into a 

 linear series of sporangia. Another variation is found in the Synchy- 

 triaceae, in which sorus formation takes place. 



2. Entophlyctis type. — In the Entophlyctis type (Fig. 3 B, p. 48), 

 the endobiotic tip of the penetration tube remains filamentous, elon- 

 gates as it drains the cyst of its contents, and lays down within the sub- 

 stratum the main axes of the branching rhizoidal system. The elements 

 of the vegetative system just beneath the substratum wall then expand 

 to form the rudiment of the reproductive structure. In the majority 

 of the fungi exhibiting this sequence of development the empty cyst 

 and the penetration tube disintegrate and play no further part. Thus 

 the incipient reproductive structure arises secondarily 1 by expansion 

 of those elements of the vegetative system immediately beneath the 

 wall of the substratum, often including portions of the primary branches. 

 Both vegetative and reproductive parts are, therefore, developed en- 

 dobiotically. The zoospores reach the outside medium by the formation 

 of a discharge tube, as in the previous type. 



Since here, as is not true of the Olpidium type, structures of a purely 

 vegetative nature (the rhizoids) are formed, as well as a reproductive 

 rudiment, this and succeeding types of thalli are said to be "eucarpic," 

 that is, they are differentiated into sterile and fertile portions. If, as 



1 Observations on the development of Endochytrium operculatum by Hillegas 

 (1940: 9) indicate that the rudiment may sometimes be formed first. 



