14(3 BOTANICAL GAZETTE. 



suppose that at length the Fungus derives too little benefit to be 

 able to go on; or the season during which the host plant flourishes 

 is drawing to an end. 



No doubt we have in heteroecism the salvation of such a Fun- 

 gus. Not only is it carried through a dangerous period, by seek- 

 ing relief at the hands of a second host, but — and which I believe 

 to be far more important — it obtains reinvigoration by the new 

 protoplasm with which it comes in contact. We may not inaptly 

 compare the sojourn of the Fungus on its second host, to a trip 

 to the seaside, where the weary and enfeebled organism enjoys 

 fresh diet and associations for a time, which in their turn pall 

 and prepare the recipient to renew old modes of life. 



We have seen that the disappearance of the sexual organs, 

 leading to apogamy, commences especially in the lower Asoomy- 

 cetes, and it may be more than a coincidence that epiphytic forms, 

 which show a tendency to produce one kind of spore while on 

 the living leaf and develope their asci on the fallen leaf are com- 

 mon here ; such forms suggest how the parasitism and heteroecism 

 of higher forms may have begun, and it is remarkable that the 

 apogamy becomes more and more complete as we ascend through 

 the latter. 



It is not pretended that the hypothesis embodied above at 

 once explains all the cases possible, and it will be well to state a 

 few of the difficulties at once. The Basidiomycetes I shall not 

 dwell upon, since our knowledge of them is still very imperfect. 



The difficulty may suggest itself to many that there are para- 

 sitic fungi — such as the Peronosporece — which nevertheless de- 

 velop the sexual organs in the condition typical and perfect for 

 the group to which they belong. I have already referred to the 

 fact that many of these forms are really saprophytes, and that 

 others break down and destroy the tissues of their hosts — clum- 

 sily killing their prey, and then feeding on the rotten mass — and 

 have pointed out that this is a much less specialized form of para- 

 sitism than that of the higher Fungi and Ustilagineoe. 



Nevertheless, the sexuality shows signs of disappearance in 

 extreme members. De Bary 1 shows that in Phytophthora and 

 Peronospora there is a less evident passage over of protoplasm 

 from the antheridium to the oosphere than in Pythium ; and that 

 in some cases, indeed, the quantity passing over is too small to 

 be observed. I will not attempt to lay stress on the coincidence 

 that in Phytophthora infestans (the fungus of the potato disease) 

 no sexual act has yet been discovered. 



1 Beitr. zur Morph., etc., der Pilze, iv, p. 72. 



