82 COMPARATIVE MORPHOLOGY OF FUNGI 



germination and in P. proiiferum and P. paimivorum, germination with 

 a hypha which ends in a sporangium (Fig. 45, 15). 



In Phytophthora two groups may be distinguished in the structure of 

 the sexual organs. The P. cactorum group, including P. Nicotianae, 

 agrees entirely with Pythium debaryanum, i.e., an antheridium is placed 

 at the side of an oogonium into which it forces a fertilization tube. 

 These forms are called paragynous. The P. infestans group, including 

 P. erythroseptica, P. Phaseoli, P. parasitica and P. Arecae, shows a new 

 structure, unique in the oomycetes; in it the oogonium grows through 

 the antheridium so that the latter sits like a collar on the oogonial stipe. 

 These forms are called amphigynous. Both types, however, are not so 

 fundamentally different that they may be separated generically; thus 

 in P. Syringae and P. Fagi occasionally amphigynous antheridia appear 

 among the paragynous (Himmelbaur, 1911; Lafferty and Pethybridge, 

 1922). 



The life cycle may be discussed for a representative of the amphi- 

 gynous group, P. erythroseptica (Pethybridge, 1913; Murphy, 1918). 

 Its antheridium always arises earlier than its oogonium, and generally 

 as a lateral outgrowth of a hypha from which later, as the material is 

 used up, they are abjointed. In youth they contain 8 to 10 nuclei. 

 Although the species is homothallic the oogonia arise on another hyphal 

 branch and generally laterally. When an oogonium fundament comes 

 in contact with an antheridium, in a short time, probably within a few 

 hours, it grows through it. The fundaments which find no antheridia, 

 cease their growth, except in P. infestans which develops partheno- 

 genetically (Pethybridge and Murphy, 1913). Outside the antheridium, 

 the oogonial fundament swells up to an oogonium (Fig. 51, 6). The 

 strong stream of protoplasm flows out of the sporiferous hyphae into 

 the oogonium until the hypha is almost empty; thus during this time 

 the oogonium contains 90 to 100 nuclei. Generally it is not separated 

 from the sporiferous hypha by a wall; but later the connecting passage 

 is closed by a plug of highly refractive material. 



Two-thirds of the nuclei degenerate both in oogonia and antheridia. 

 Thereupon they arrange themselves peripherally, with one in the center 

 of the oogonium and undergo a single simultaneous division. All 

 peripheral nuclei and one daughter of the central nucleus degenerate 

 (Fig. 51, 7). Meanwhile there has collected around the central nucleus 

 the somewhat denser ooplasm, while the periplasm remains almost 

 structureless. It is entirely resorbed by the ooplasm, so that shortly 

 after fertilization only the remains of the degenerated nuclei are present. 

 Meanwhile in the antheridium the nuclei have also undergone division 

 and all but one disintegrate. Then the oogonial wall is dissolved gradu- 

 ally in one spot (receptive spot). Because of its higher osmotic pressure, 

 the antheridium pushes a short fertilization tube into the oogonium (Fig. 



