74 INTRODUCTION TO THE STUDY OF FUNGI 



of its vegetative vitality, and produces here and there external 

 nodules, which are the commencement of efforts towards repro- 

 duction. In a comparatively short period of time the surface 

 of the log is decorated with young Agarics, effused patches of 

 Corticium or Porta, or other forms of Hymenomycetal Fungi. 

 Fully thirty-five per cent of the known species of Agarics are 

 developed in this manner from decaying wood and leaves ; and 

 of the remaining sixty per cent, allowing five per cent for other 

 contingencies, it may fairly be assumed that a large proportion 

 flourish at the expense of the dead roots, vegetable humus, 

 indistinguishable fragments and remains of vegetation which 

 are present in the soil, or the dung of animals which are 

 vegetable feeders. If we turn from the Agaricini to other 

 of the great groups of the Hymenomyceteae, such as the Polyporei 

 and the Thelephorei, we shall be still more convinced of the 

 great, almost overwhelming, preponderance of species which 

 manifestly flourish upon the remains of previous vegetation. 

 Of the 9600 known species of Hymenomycetal Fungi, really 

 parasitic species are almost, if not wholly, unknown. 

 ^ Although numerically inferior to the saprophytes, the 

 parasitic Fungi are none the less important, from the sad havoc 

 they are capable of producing amongst cultivated plants. A 

 little experience will soon demonstrate that the parasites are of 

 two kinds — namely, those which establish themselves externally 

 upon the green parts of growing plants, and do not penetrate 

 the tissues ; and those which are developed internally, deeply 

 seated in the tissues of the infested plants, and not making 

 their appearance externally save for the purpose of fructifica- 

 tion. The former may be called epiphytal, growing upon plants, 

 and the latter endophytal, growing within plants, which may be 

 illustrated by familiar examples. Common epiphytal parasites 

 are — the various species of the Erysiphei ; such as the Sphaero-' 

 theca castagnei, or hop mildew ; the Sphaerotheca pannosa, or rose 

 mildew ; the pea mildew, or Erysiphe Martii ; the mildew of 

 the maple, Uhcinula bicomis ; and many others. These all 

 appear upon the leaves of living plants in thin effused white 

 patches, which give the appearance of being dusted with flour. 

 There are two stages or conditions of the parasite, and both 

 associated with the web -like mycelium. The earliest stage 



