438 Prof. H. Marshall Ward. The Relations between 



I ongM to add, that if a potato plant is grown in a pot and kept 

 under a hell-jar (untouched by Phytophthora) normally lighted, in the 

 summer, the excessively watery dark-green shoots often develop 

 hump-like outgrowths, composed of very large, thin-walled cells, 

 which may be regarded as due to the excessive turgescence and 

 hypertrophy of these cells. Presumably they contain relatively large 

 quantities of organic acids, &c, and everything indicates that such a 

 shoot would easily succumb to the Phytophthora, as in fact it does. 



Kiihn long ago noticed* that there are two periods when the potato 

 shoot is most easily infected by the Phytophthora. The first is while 

 still young — fully developed internodes are much more difficult to 

 infect than young growing ones, a fact well known and easy to con- 

 firm ; the second period is said by Kiihn to occur after the tissues are 

 far advanced, at the end of July or the beginning of August, and 

 this would seem to be borne out by the experience of cultivators 

 generally. I am inclined to regard this second period as coincident 

 with the time when the plant is particularly rich in the products of 

 assimilation on their way down to the tubers. They travel chiefly as 

 glucose, and one consequence of the abundance of this carbohydrate, 

 and the increased metabolism it supports, is an increase in the 

 organic acids. If wet and dull weather sets in when the tissues are 

 thus, so to speak, overflowing with such substances, the Phytophthora 

 is peculiarly favoured, and can spread through the plant with the 

 rapidity characteristic of an epidemic. In the allied genus Pythium, 

 the phenomena are so similar that we may assume that the fungus 

 behaves like Peronospora : the species are often saprophytes, however. 



The question now arises, can these ideas be extended to the case of 

 other parasitic fungi ? It would be difficult to say with regard to the 

 Saprolegnieai and the Mucorini, because so little is known of their 

 parasitism. As regards the Ascomycetes generally, we may expect 

 great differences in respect to types like the Gymnoascece, Rhytisma, 

 Hysterhim, the JErysiphew and the Sphcerias, and they certainly 

 occur. 



Some Nectrias at any rate (if e.g., N. cucurbitula, N. cinnabarina, and 

 N. ditissima) behave so differently towards the host that we may 

 probably conclude that the mode of procedure is unaffected by such 

 variations on the part of the latter as have been sketched; and the 

 same may be said of the wood-destroying Hymenomycetes (e.g., Agari- 

 cus melleus, Trametes radiciperda, Polyporus sulphureus, &c). 



In all these cases the tree, as a whole, suffers in an indirect 

 manner : these various cankers and rots, &c, destroy, for the most 



* ' Ber. aus dem Physiol. Lab. u. d. Yersuchsanstalt des Landw. Instituts d. Univ. 

 Halle,' 1872, pp. 81—82, quoted by Sorauer, vol. 1, p. 140. 



f 'Unters. a. d. Fo.-stbot. Inst, zu Munchen,' vol. 1, pp. 88 and 109, and vol. 3 

 also K. Gothe (Thiel's * Landw. Jabrb.,' 1880, vol. 9, p. 837). 



