FUNGT. SAPROLEGNIEAE. 



99 



travels on to the second, and so on. The granules in the protoplasm of the oosphere 

 withdraw in many cases from the spot where the tubes touch the oospheres, but 

 the extremity of the tube in contact with the oosphere always appears to be perfectly 

 closed; there is no opening to be seen in it and no passage of the contents of the 

 antheridium into the oosphere, as in Pytliium and other cases. Nevertheless the 

 oospheres show the usual signs of maturation ; they become invested, for instance, with 

 a wall of cellulose which eventually is differentiated into an exosporium and an endo- 

 sporium. The same course of things is seen in Achlya prolifcra and A.polyandra as in 

 Saprolegniaferax. In other individuals of Saprolegnia the antheridium becomes firmly 

 attached to the wall of the oogonium, but no fertilisation-tubes are formed, or they do not 

 reach the oospheres. Finally, there are found not a few individual plants of Saprolcgnia 

 fera.v, S. asterophora, Achlya spinosa and Apkanomyces, on which no antheridia are 

 formed, but in which the oospheres pass without the co-operation of antheridia through 

 all the stages of development observed when antheridia and fertilisation-tubes are present 

 with them. It appears then that a process of fertilisation, such as takes place in the 

 Peronosporeae and most distinctly in Pythiiim, does not occur in the Saprolegnieae, 

 and that this is a group of plants which has lost the power of sexual propagation ; 

 the phenomena of conjugation having disappeared, they have fallen into the conditon 

 of apogamy. And if we consider the Peronosporeae and the Saprolegnieae in series 

 together, we have a highly interesting view of the way in which this condition has been 

 gradually introduced ; it is here given in de Bary's own words. 



1. One end of the series is formed by species of Pythiiim, in which the larger part 

 of the protoplasm of the antheridium passes over as gonoplasm into the oosphere, 

 when an opening in the delicate cell-wall of the fertilisation-tube has established 

 a communication with the oosphere. In other words, conjugation takes place between 

 the oosphere and the contents of the antheridium. 



2. In Phytophthora a very minute portion of protoplasm, but one that can still be 

 followed with the eye, passes through the fertilisation-tube from the antheridium to the 

 oosphere. In this case too there must be a narrow opening in the tube. 



3. In Peronospora the presence of such an opening is not to be directly recognised, 

 nor can the protoplasm of the antheridium be distinctly followed in its passage into the 

 oosphere. But the complete agreement of the other points observed in this case with 

 what has been ascertained in the case of Phytophthora makes it highly probable that the 

 passage of a very minute portion of protoplasm does take place in Peronospora also. 

 Whether the protoplasmic matter passes through a narrow or a wider opening in the 

 wall of the tube, or by osmosis through the interstices of its micellar structure, must for 

 the present remain unsettled. 



4. In certain species or individuals of Saprolegnia, Achlya z.n&Aphanomyccs there is 

 close contact between tube and oosphere, but there is no apparent opening or passage 

 of the contents of the antheridium into the oosphere. 



5. There are other individuals of Saprolegnia, as 5. torulosa and S. asterophora, in 

 which the antheridium becomes firmly attached to the wall of the oogonium, but either no 

 fertilisation-tubes are formed, or those which are formed do not reach the oospheres. 



6. Finally, oogonia and oospores are produced without the formation of antheridia. 

 The germination of the oospores agrees with that which has been given above for the 



Peronosporeae, that is, the oospore with all its protoplasm first forms a short germ-tube 

 and then becomes a zoosporangium ; or all the protoplasm passes into the germ-tube, 

 which is then cut off by a transverse wall and becomes a zoospoiangium, but may if 

 sufficiently well fed ramify and form several typical sporangia and then disappear ; 

 or, lastly, the germ-tube developes without a previous formation of zoosporangia into a 

 vegetative thallus, which only when it has reached its normal form and size produces 

 zoospores and oogonia. Achlya polyandra and various species of Saprolegnia show 

 all three modes of germination according to circumstances ; Achlya spinosa has only 

 the third, Phytophthora omnivora only the second, &c. 



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