NATURAL AND ARTIFICIAL INFECTION. 53 



the stomata. Also, whether the germ-tube formed from a 

 germinating spore penetrates direct, or if, as shown by De Bary 

 for Sclerotinia, a mycelium vigorous enough to penetrate must 

 first be developed saprophytically. 



In this connection De Bary ' states that the germ-tubes from all aecidio- 

 spores and uredospores only penetrate by stomata, and thence extend through 

 the intercellular spaces. Entry through the stomata has also been observed 

 on the germ-tubes from sporidia of Leptopuccinia dianthi, and from spores of 

 Entyloma. On the other hand, germ-tubes from the spores of teleutospores, 

 from spores of Peronosporeae, Ustilagineae, Sclerotinia, Polystigma, Pro- 

 tomyces, a.nd Syrvehytrium effect an entrance through the outer cell-walls into 

 the epidermal cells or stomatal guard-cells. De Bary also describes the 

 peculiar behaviour of zoospores of Cystopus and Peronospora umhelliferarum, 

 which, if they come to rest near a stoma, germinate, and the germ-tube 

 enters therein, whereas one developed in water soon dies. Certain fungi 

 penetrate sometimes through the membrane, sometimes by a stoma, e.g. — 

 Phytophthora infestans, Peronospora parasitica, Exohasidium vaccinii. 



In the case of Phytophthora omtiivora, Hartig found that the germ-tubes 

 from the zoospores crept along the surface of the leaf till they reached a 

 place where two epidermal cells adjoined ; there they entered, and only 

 rarely grow into the epidermal cells. The germ-tubes of Protomyces macro- 

 sporus and Tuhurcinia Irientalis enter their hosts in the same way. 



From experiments, one is able to determine the conditions 

 favourable, or otherwise, to infection by parasitic fungi; to ascertain 

 the influence of temperature, air-moisture, water-content of the 

 host, hairiness of the leaves, and the effect of resin or other 

 excretions as protections to wounds. For example, it was in 

 this way that Hartig found Salix pidchra (pruinosa x daphnoides) 

 to be a hybrid which, on account of its hairy leaves, is more 

 resistant to Melampsora than Salix pruinosa.'^ Much investiga- 

 tion remains yet to be done in this direction to ascertain what 

 varieties or species of cultivated plants are likely to be least 

 liable to attack by epidemic diseases.^ 



The methods used in carrying out artificial infection are 

 based on the observation of cases of natural infection. Most 

 frequently infection is performed by means of spores, less often 

 with mycelium. 



The spores of lower forms of fungi are generally distributed 

 by means of water, especially in dew or rain. Zoospores are 



^ Morphology and Bioloqy of the Fungi, English Edition, pp. 361-362. 

 2 Hartig, Diseases of Trees, English Edition, 1894, p. 171. 

 ^See Chapter v. on "Disposition." 



