CONTRIBUTIONS 1'KOM THE WISLEY LABORATORY. 



509 



been attracted to grow in the leaves of the apple, in much the way shown 

 by Massee, and alluded to above, is afforded by the fact that in certain 

 plants, when they are exposed to the action of low temperatures for a 

 time, other substances than those produced when the temperature is at 

 the optimum for development are produced. It is well known, for 

 instance, that starch may be converted into fatty oils in plants placed 

 under these conditions, these oils being reconverted into starch when the 

 more appropriate temperature recurs. In many conifers red drops of 

 oil are produced in the leaves only during the cold weather, and the leaves 

 therefore acquire a reddish tint, regaining their normal green colour 

 when the oil disappears in the next growing season. When starch is 

 the form of food stored, this food is frequently converted into glucose 

 when the temperature falls to about 36°, and remains in that form until 

 the recurrence of higher temperatures. This is well seen in the potato, 

 which becomes quite sweet when stored at a temperature between 32° 

 and 42°, owing to the production of sugar ; but when the temperature 

 is raised above 50° this sugar is reconverted into starch.* 



Whether a similar accumulation of sugar occurs in the leaf of the 

 apple is not yet known, but this seems probable when the leaves are 

 exposed to a low temperature for a length of time. If so, the fungus is 

 no doubt attracted to the leaf on this account, as most fungi have been 

 shown to be attracted by the presence of sugar. In any case the reason 

 of the attack is probably that, owing to the unusual climatic con- 

 ditions under which they were growing, some substance has been formed in 

 the leaves which in normal years is not formed at all, or at any rate to a 

 very small extent, or that, from the same cause, some substance which has 

 the power of repelling the entrance of the germ tubes of the fungus has 

 failed to be formed. 



Other Conditions contributing to the Attack. — It seems probable that 

 other conditions besides unfavourable weather may at times lead to the 

 attack of this fungus. For instance, in one case the apple 1 Stirling 

 Castle ' has been attacked two or three years in succession and had, 

 apparently, previously been weakened by excessive cropping, little growth 

 having been made in each year. A correspondent says that year after 

 year the apple ' Cox's Orange ' and other thin-leaved varieties have been 

 attacked. Another correspondent (Mr. Spencer Pickering, F.R.S.) says 

 that both at Ridgmont and at Harpenden any tree that is in a weakly 

 condition (such, for instance, as those recently transplanted) has been 

 " so bad that the whole of the foliage is absolutely brown. . . . We are 

 never entirely without this trouble." 



Varieties Attacked. — It is very instructive to observe the difference 

 in the amount of damage sustained by different varieties growing under 

 identical conditions. In all districts ' Cox's Orange ' appears to have 

 suffered the most severely, and in some gardens this variety was the 

 only one to suffer to any considerable extent. It was not, however, by any 

 means the only one to be attacked in other places. At Wisley some 

 175 varieties of apples are planted, and of these very few escaped. 

 1 Cellini Pippin,' ' Cox's Pomona,' ' Diamond Jubilee,' ' Endsleigh 

 Beauty,' 'Foster's Seedling,' ' Grantonian,' 1 Hormead Pearmain,' 

 * See Pfeffer, PJiysiology of Plants, i. p. 512. 



