EFFECT OF PARASITIC FUNC4I ON THEIR HOST. 17 



perceptible disturbing effect on the liost. Such is the case 

 particularly with leaf-spot diseases ; the tissues of isolated spots 

 are killed and fall out, the leaf appearing as if perforated by 

 shot, but otherwise exhibiting no discoloration or other symptom 

 of disease. In contrast to these there are fungi w^hich, directly 

 or indirectly, bring about death of their host or some part of it. 



The simplest example of parasitic fungi killing their host 

 directly is presented by one-celled or few-celled plants, which 

 soon succumb to attack even on a single cell. Where, however, 

 the host is a highly organized plant, its organs will resist the 

 attack of the parasite for some time. Thus with Phytophthora 

 fagi, the mycelium spreads rapidly through the tissues of a 

 seedling, so that death ensues in a few days. Similarly species 

 of Peronospora rapidly kill leaves, branches, and fruits ; likewise 

 Cladosporium , Sqytoria parasitica, and others. 



Somewhat different in their action are those fungi which 

 kill some tender part of a plant directly, and thereby in- 

 directly further the death of other parts dependent thereon. 

 As examples, take Pcstalozzia Hartigii (Fig. 301) and Plioma 

 abietina (Fig. 293), which kill only some small portion of a 

 young plant or branch, but thereby cause drying-up of higher 

 or distal parts. Gibhcra vaccinii on stems of cowberry (Fig. 95) 

 is another example. Similarly cankers arising from Nectria 

 ditissima (Fig. 80), or Peziza Willkommii. Again, Agaricus 

 mcUcv.s and Trametes radidperda kill roots or lower portions of 

 the stem, and bring about the death of trees of all ages. 



The case varies somewhat with certain wound-parasites like 

 Nectria cinnalarina and Cucnrhitaria lahurni. There the my- 

 celium extends so vigorously in the water-conducting organs, 

 as to kill them and fill up the vessels, causing thereby so 

 serious a disturbance in conduction, that branches or whole 

 plants wither away in summer. The wood-destroying Pohjporcac 

 and Agaricini act similarly, although more slowly ; they attack 

 large branches and stems, destroying all parts of the wood, 

 duramen as well as sap-wood, and finally the bark. 



There are also cases where organs of the attacked host 

 remain alive, but suffer on account of the hypertrophy of other 

 parts. In this way portions of a plant may be killed although 

 not directly the seat of the parasite. This is particularly the 

 case where hypertrophied organs undergo increased growth and 



p. 



