DISEASES OF PINES. 59 



more important is the pressing into the interstices of the drained soil of 

 atmospheric oxygen, which does so much work of various kinds in the 

 labyrinth of passages which it traverses, that a whole lecture would not 

 nearly exhaust the treatment of this subject alone. Another extremely 

 pertinent point in this connection is that the drained soil can be warmed 

 by the sun's rays, or by the higher temperature of the air referred to, 

 not only more easily, but also more equably. 



Passing now to the diseases due to unsuitable conditions in the sub- 

 aerial and atmospheric environment, the following points may be considered. 



Pines, especially when the foliage is young, and still more particularly 

 when the plants themselves are young, are apt to lose many leaves, and 

 even to be killed, by undue chilling of the surfaces, cold dry winds being 

 perhaps the most fatal agents in this country. I have already referred to 

 that form of leaf-casting which is caused by this ; but it is perhaps 

 commoner to see parts of the tree only, in the case of the more tender 

 Pines, with their foliage brown and shrivelled, than to have a general fall 

 of the leaves. 



A curious class of diseases, not common in the Pines, perhaps, but 

 stated as occurring in P. Strobus and some others with thin cortex, are 

 the various kinds of "rifts" i.e., more or less vertical fissures -which 

 extend up and down the exposed trunks of trees facing the south-west. 

 The particular kind of 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 rejecting 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. 



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 j 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, Uredineae, and Hymenomycetes. It is true that Phytophthora 

 omnivora (one of the Peroiiosporeae) attacks and destroys the seedlings of 

 these and other Conifers ; but the rule is that Conifers are exempt from 

 diseases due to the Peronosporese, Ustilagineae, Gymnoascese, or Gaster- 

 omycetes, and also from those caused by Bacteria (possibly with one 

 exception*) and Myxomycetes. 



* Vuillemin, " Sur une Bacteriocecidie ou Tumeur Bacillaire du Pin d'Alep," Comptes 

 Hendus, November 26th, 1888. It may also be remarked that the roots of certain Conifers 

 may have hyphre of Gasteromycetes attached to them, though, so far as I can discover, 

 they do not induce diseased conditions in the tree 



