4 THE PARASITIC FUNGI. 



but as a rule they live throughout their whole development as 

 parasites. 



Within each of these four divisions one may introduce a 

 number of subdivisions. 



Hemi-sapropbytes. 



The majority of saprophytes are never parasitic, yet there are 

 a number which become so occasionally. Thus some species of 

 Mimm- and Penicillium can penetrate into thin-skinned fruits, and 

 this they do the more easily, the further the fruits are from the 

 condition of full vital energy, to use De Bary's expression.^ 

 Eelated to these are other fungi which, although incapable of 

 effecting entrance into plants in active life, may yet do so as 

 the plant, though still living, begins to wither. In such cases 

 the parasitism is somewhat difficult to prove. In particular, the 

 so-called ' Fungi imperfecti ' contain forms of this kind. 



Amongst the hemi-saprophytes we may include the species 

 of Botrytis, which are able to penetrate into unfolding parts of 

 plants, but not into the older parts. We may specially mention 

 Botrytis Bouglasii as a form more generally known as a sapro- 

 phyte, but which becomes parasitic on immature organs, and 

 which penetrates young needles of various conifers to kill them, 

 whereas it is unable to attack older needles. In this case the 

 thickness of the membranes would seem to act as a protection, 

 just as the vital energy of the plant does in the preceding eases. 

 In Sclerotinia sclerotiorum, Scl. dborioides, and Scl. Ihtckeliana, 

 a saprophytic existence must, as in the example just mentioned, 

 precede the parasitic condition ; in fact De Bary holds that 

 these forms can only become parasites after their mycelium has 

 been saprophytically strengthened; the parasitic condition is not 

 necessary to them, for they can go through their whole develop- 

 ment on a dead substratum. Bythium Be Baryanuin is also to 

 be regarded as a hemi-saprophyte which attacks and kills 

 seedlings of many plants as a parasite, but otherwise vegetates 

 on dead plant remains. Cladosporium herbarum, one of the 

 commonest of saprophytes, behaves similarly, but it is of less 

 frequent occurrence than Pythium, and in fact its parasitism has 

 only been suspected quite recently. 



^This has been confirmed bv Davaine (Compt. rend. Lxm., 1866, pp. 277 and 

 344) and Brefeld {Sitzungsber. d. nalurforsch. Fr. zit Berlin, 1875). 



