CHAPTER V. — COMPARATIVE REVIEW. — PERONOSPOREAE. 



137 



and especially of genera, may be taken chiefly from the gonidial formations, while the 

 few species which have no gonidia are not easily classified. 



The main features in the formation of gonidia in the genera and subgenera of the 

 Peronosporeae are as follows : — 



Fythium. A persistent cell, usually the terminal cell of a branch, is delimited by a 

 transverse wall and becomes a spore-mother-cell (sporangium). The gelatinously 

 thickened wall at its apex suddenly expands into a thin-walled spherical vesicle, and 

 into this at the same moment the whole of the protoplasm of the cell, which is hitherto 

 undivided or has only shown transitory beginnings of division, streams rapidly, within a 

 few minutes' time at most ; there it breaks up at once into a number of swarm-spores, 

 which issue from the delicate swelling vesicle and finally germinate. The sporangia in 

 some species are of the same form as the gonidia of Phytophthora (Fig. 64), — round or 

 ovoid vesicles prolonged at the extremity into a neck or beak, in the apex of which the 

 swarm-spores are formed ; in others any portion, and often a very long portion, of the 

 cylindric filamentous thallus-tube is delimited to form the sporangium, and its apex in 

 which the swarm-cells are formed is then a small knob-like enlargement at the 

 extremity of a branch, but is 

 not otherwise distinguished by 

 any particular form. There is 

 usually no strict regularity dis- 

 cernible in the arrangement of 

 the sporangia. In species with 

 vesicular sporangia the fila- 

 ment which bears them often 

 grows on into the empty spor- 

 angium from the point of its 

 insertion, or through its entire 

 length, and then forms a new 

 terminal sporangium as in 

 Saprolegnia (see p. 46). A 

 certain kind of regular ar- 

 rangement and succession oc- 

 curs in one species only, 

 Pythium intermedium, and in 

 two ways, sporangia being either formed sympodially and successively on a branch 

 of the thallus and separated by elongated portions of the thallus, as is described on 

 pages 47 and 65, or by serial successive abjunction (see p. 66). In the same species 

 the sporangia with their wall of delimitation are easily and abundantly shed from their 

 sporangiophore, so that they may equally 

 well be called spores, and the name is 

 still further justified by the fact that 

 under certain circumstances they may 

 put out a germ-tube immediately without 

 forming swarm-spores. P. de Baryanum 

 also often forms spores (gonidia), which 

 have the same form as the sporangia, 

 in place of sporangia, but germinate 

 directly by the emission of a tube. 



Phytophthora (Figures 64, 65). 

 Branches of the thallus either solitary or 

 growing side by side in small tufts con- 

 stitute peculiar gonidiophores and form gonidia. They generally send out a few branches 

 monopodially disposed, and each of these secondary branches ,or the unbranched 

 gonidiophore forms a number of gonidia sympodially and successively at elongated 



FIG. 64. Phytophthora infestans. Extremities of two simple sporophores. a for- 

 mation of the first gonidia on the tip of each branch, b two ripe gonidia on each 

 branch, a third beginning to form. Magn. about 200 times. 



FIG. 65. Phytophthora infestans. a sporangium in water 

 after the division is completed, b escape of the ten swarm-spores 

 from the sporangium, c spores in the motile state, d spores come 

 to rest and beginning to germinate. Magn. 390 times. 



