130 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



rift here referred to rarely, if ever, appears in trees grown in the 

 open from their youth onwards, but is very apt to occur on the 

 south-west aspect if older trees previously closed up and well 

 sheltered are exposed by a cutting. I see no reasons for re- 

 jecting the explanation that such rifts are caused by the direct 

 rays of the sun beating on the thin cortex when the air is at 

 its highest temperature; whether the cells are killed directly 

 by the sun's rays, or whether the damage is due to excessive 

 evaporation of their water, is as yet not certain. 



I must be contented with the mere reference to these 

 phenomena here, however, and with the remark that difficulty 

 is experienced in distinguishing between these sun-rifts and 

 other cases of splitting, or even wholesale desiccation of the 

 cortex of the more tender Pines owing to sudden drought, frost, 

 cold winds, &c., to which they may be suddenly exposed by the 

 removal of neighbouring trees which sheltered them previously. 



Of all the sub -aerial agents which damage Pines, however, none 

 are perhaps more to be feared than the acid gases of our larger 

 manufacturing towns. Sulphurous acid, hydrochloric acid, 

 chlorine, coal-gas, and such-like chemicals are fatal to Pines 

 even in very small quantities ; and it is no doubt to these, rather 

 than to the increased percentage of carbon dioxide, soot, or to 

 the diminished light, that the foggy exhalations of large towns 

 owe their enormous power for evil. Nor can we wonder at this 

 when we reflect that many Pines are mountain species, growing 

 normally in those purest of atmospheres which attract us for the 

 very reason of their purity. 



I now pass to the consideration of those diseases of Pines 

 which are directly traced to the injurious action of fungi on or 

 in their roots, stems, or leaves. 



These fungi belong almost exclusively to the groups of 

 parasitic Ascomycetes, Uredine^, and Hymenomycetes. It is 

 true that PhytoioMliora omnivora (one of the Peronosporeae) 

 attacks and destroys the seedlings of these and other Conifers ; 

 but the rule is that Conifers are exempt from diseases due to the 

 Peronospore^e, Ustilagine^e, Gymnoasceffi, or Gasteromycetes, 

 and also from those caused by Bacteria (with one exception*) 

 and Myxomycetes. 



* Vuillemin, " Sur une Bact6rioc6cidie ou Tumeur Bacillaire du Pin 

 d'Alep," Coin;ptcs Eeiidics, November 26, 1888. It may also be remarked 



