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JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



(6) This lack of bright sunshine, together with the excess of rainfall 

 for the period (3*64 inches, against 2*76 in 1906, and 0*96 inch in 1905), 

 would have the effect of keeping the atmosphere more moist than usual 

 and of checking transpiration. That a very considerable amount of 

 water vapour was held in the air during a great part of the period may 

 be seen by comparing the temperatures recorded by the wet- and dry-bulb 

 thermometers in the accompanying temperature chart (fig. 90). 



(7) Although, as shown above, the minimum temperature never fell 

 low enough to cause the leaves to die, yet the thermograph record shows 

 that during this period of forty-two days the temperature was below 

 42° (a temperature that may be regarded as one at which growth and 

 assimilation may proceed but very slowly, and below which little activity 

 is manifest) for more than 154 hours, that is, nearly one-sixth of the 

 whole time. This checking of the growth for so long a time must have 

 resulted in the somewhat abnormal development of the leaves. 



(8) Cold, dry northerly and easterly winds, especially when 

 blowing at a rapid rate, often do more harm to vegetation than many 

 frosts unaccompanied by wind. Unfortunately for our purpose the 

 anemometer at Wisley is not constructed to make graphic records of the 

 rate and direction of the wind so that these may be known for any period 

 of the day. Only the direction of the wind at 9 a.m. is recorded, 

 together with the distance registered by the anemometer for the twenty- 

 four hours. These data are insufficient for an examination into the 

 particular point under discussion, and may be misleading. However, so 

 far as they go, they do not show any prevalence of strong, dry winds 

 from these points of the compass. The strongest winds came from the 

 south-west, and these were generally accompanied by rain. 



Thus it appears that some of the weather conditions were such as to 

 lead to abnormal development of the foliage, viz., a deficiency of bright 

 sunlight, a considerable deposition of moisture, a high relative humidity, 

 and a continued period during which low temperatures prevailed, and 

 certain of the vital processes were almost at a standstill. These com- 

 bined seem to have altered the leaf, so that it was open to the attack of a 

 fungus. 



The Fungus implicated. — As stated above, a fungus was found in 

 every case of leaf spotting of the type described, and the fungus in every 

 case was Cladospwium herbarium. This fungus is exceedingly common 

 upon dead vegetable matter of various kinds, growing as a saprophyte. 

 But, like several of the fungi that usually grow upon dead vegetable 

 matter, it has the power, in certain circumstances, of becoming a 

 parasite and causing the death of living tissues. Thus, not infrequently, 

 this fungus has been the cause of a leaf-spot and pod-spot of peas ; poppy 

 capsules have been attacked ; Kosmahl • has attributed the death of 

 seedlings of Pinus rigida to this species ; Cavara has found it as a 

 harmful parasite on raspberries (on which host I have also found it 

 doing considerable damage), Cycas, Agave, and other garden plants. 

 Delacroix and Prillieuxt have described it as injuring apples, and it is 



* Ber. d. deutsch. bat. Ges. x. p. 442. 



+ Wall, de la tioc. Mycol. de France, vi. (1890), p. 136.— These investigators say: 

 " Le cas le phis important est celui des ponnnieis, qui, il y a deux ans surtout, en 



