CONTRIBUTIONS FROM THE WISLEY LABORATORY. 503 



(8) the scorching of the foliage through the hot sun shining directly 

 upon it after rain or heavy dew. 



As pointed out above, the portions of the shoots that suffered most 

 from the attacks of the fungus were usually the first nine inches to 

 twelve inches of the growth. This being so, we may assume that if 

 this spotting were directly due to adverse climatic conditions, these must 

 have occurred during the time when the growth was made, that is, during 

 the latter end of April or the month of May. The screen in which the 

 maximum and minimum thermometers are kept is about the height of 

 the branches of many of the bushes from the ground, and we may take 

 the minimum recorded in the screen as the minimum to which the vast 

 majority of the bushes were exposed. An examination of the temperature 

 chart which accompanies this paper (fig. 90) shows that the minimum 

 temperature during these six weeks did not once reach the freezing- 

 point. Even on the grass, where the radiation is intense (and the trees 

 are in cultivated ground), the thermometer only six times touched 

 freezing-point and only twice fell a little below 30°. It is inconceivable 

 that the spotting of the leaves of such a hardy plant as the apple should 

 be due to such a small amount of frost. The justice of this inference is 

 upheld by the fact that in previous years, of which we have any record 

 here, the trouble was not nearly so rife ; when it was, indeed, so slight 

 as to call for no remark, frosts were much more prevalent. The 

 thermometer in the screen during the same period in 1904 did not touch 

 freezing-point, in 1905 "it fell to 32° once, while in the same period in 

 1906 seven frosts were recorded. The difference on the grass is more 

 marked ; for while in 1907 the thermometer reached the freezing-point 

 six times, frost was recorded during the same period in 1904 nine 

 times, in 1905 fourteen times, and in 190G twelve times. It would seem, 

 then, reasonable to conclude that frost could not have been the direct 

 cause of the trouble. 



If we turn to the prevalence of low temperatures we find a similar 

 state of things. The minimum temperature in the screen fell during 

 the period of six weeks four times ' below 36° ; in 1904 it fell 

 to that temperature five times ; in 1905 six times ; and in 1906 ten 

 times. On the grass, a temperature of 36°, or below, was recorded twelve 

 times during the period ; in 1904, however, fifteen times ; in 1905 

 twenty times ; and in 1906 sixteen times. Here again, therefore, the 

 season of 1907 does not seem to have been worse than the few pre- 

 ceding ones, as far as temperature goes ; and it would not appear just to 

 ascribe the prevalence of the leaf spotting to the direct action of low 

 temperatures. 



The question of sun scorching is less easy to investigate after a short 

 period of time than the foregoing ; but as the leaves affected were quite 

 as frequently on the north side of the tree, as on the other sides where 

 they would be* directly exposed to the sun, and perhaps more frequently 

 in the middle of the tree than on the outside (but that partly arises from 

 the fact that the earlier part of the growth was usually most injured), it 

 would appear that sun scorching had little to do with the question. In 

 another garden at Cobham, where the trees were better protected from 

 changes of temperature and sheltered from cold winds from north and 



