CHAPTER V. — COMPARATIVE REVIEW. — PERONOSPOREAE. 



l 33 



are being formed by comparatively few transverse walls disposed at irregular 

 intervals. Many of the parasitic species put out from the flanks of the intercellular 

 thallus-tubes numerous small branches, which vary in form according to the species 

 and penetrate into the interior of the adjacent cells in the form of haustoria (Fig. 8). 

 The development of the thallus ceases with the formation of sexual organs : female 



FIG. 6i. Formation of oospores and processes of fertilisation in the Peronosporeae. /— VI. Pythium gracile. Suc- 

 cessive states of an oogonium. / mature oogonium ; to the right of it is an antheridial branch formed but not yet 

 delimited. II antheridium delimited by a transverse wall. /// the oosphere has rounded itself off in the oogonium, 

 and a thin zone of periplasm lies between the oosphere and the wall of the oogonium. IV the antheridium has put 

 out the fertilisation-tube, and a clear receptive spot is visible on the oosphere. V passage of the gonoplasm from the 

 antheridium into the oosphere. / 7 ripe oospore surrounded by a thick membrane and almost entirely filling the cavity 

 of the oogonium. VII. Peronospora arborescens ; an oogonium with antheridium attached which has put out a fertilisa- 

 tion-tube. The oosphere is already invested with a thick membrane ; outside it is a comparatively broad zone of peri- 

 plasm, which is contracting to form the exosporium round the oospore. I— VI magn. about 800, VII 600 times. 



organs 



oogonia, in each of which one oosphere is formed, and male organs, 

 antheridia, by which fertilisation is effected in the oosphere which developes into the 

 oospore. The behaviour of these organs in the process of fertilisation varies con- 

 siderably in the different genera. It shall be described first in the genus Pythium. 

 The oogonia of this genus (Fig. 61) are terminal or intercalary spherical 



a be 



Fig. 62. Sexual organs of Peronospora A Isineanim, Casp. a young state, b formation of the oosphere and 

 fertilisation-tube, c after fertilisation ; periplasm somewhat contracted by the preparation, and the fertilisation-tube 

 unusually thick in this specimen, n antheridium, oogonium. Magn. nearly 350 times. 



swellings of the thallus-tubes. Their surface remains smooth as it enlarges, or is 

 rendered spiky in some species by projecting points. Their wall of cellulose is 

 moderately stout. When they have reached their full size they are delimited from 

 the thallus by one, or, if their position is intercalary, by two transverse walls, and are 

 then filled with dense protoplasm containing numerous drops of oil (Fig. 61, /). 



