60 H. MARSHALL WARD. 



to last until the nest generation of the fungus gains its hold-fast on 

 another (and it may be distant) source of life-giving substance. 



Let us take the case of a Uredinous fungus parasitic in the leaves of 

 a phanerogam. We know that the substances necessary for the whole 

 growth of the phanerogam are formed in the cells of the leaf; not only 

 so, the matters which eventually find their place in the reproductive 

 organs must be formed there also, potentially at least. The leaf of a 

 phanerogam so attacked, moreover, is able to support the parasitic 

 fungus for a long time uninjured, as I have convinced myself by ex- 

 periment, and there can be no doubt that substances pass into the 

 fungus which would normally have passed into other parts of the host 

 plant itself. Since these substances serve to support the compara- 

 tively enormous display of energy evinced in the growth, &c, of the 

 phanerogam ; we need not be surprised if they can also provide in 

 addition for the parasite for the time being. 



But we may imagine even this to fail after a time. Without specu- 

 lating as to the possible differences effective to a mycelium which 

 obtains enough to produce spores on one leaf, which, germinating on 

 another, produce a mycelium which derives an advantage correspond- 

 ing to that obtained by plants when cross fertilised — we may 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 fungus. 

 Not only is it carried through a dangerous period, by seeking relief at 

 the hands of a second host, but — and which I believe to be far more 

 important — it obtains re-invigoration 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 sea-side, where the 

 weary and enfeebled organism enjoys fresh diet and associations for a 

 time, which in their turn pall and prepare the recipients to renew old 

 modes of life. 



We have seen that the disappearance of the sexual organs, leading 

 to apogamy, commences especially in the lower Ascowiycetes, 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 common here ; such forms 

 suggest how the parasitism and heteroecism of higher forms may have 



