398 



Prof. H. Marshall Ward. The Relations between 



within wide limits ; much more important is the amount of water 

 and air in it, and the way they are held there. These, especially 

 under certain crops, affect the climate of the immediate locality, and 

 all kinds of complexities result. To mention one only, there are 

 certain combinations of soil and climate, &c, which result in the 

 trees being " frost-bitten " whenever there are late spring frosts. In 

 some cases it is found that mere drainage puts an end to the evil ; 

 this means not only a removal of water, but an increase of air in the 

 soil and general elevation of temperature. In others it is noticed 

 that the more shaded trees suffer most ; this is in part because their 

 tissues are more watery, and their cell- walls more delicate. In others 

 the injury occurs on a particular side of the tree, and is ruled 

 chiefly by the prevailing winds. Now, here is a problem of con- 

 siderable complexity. Frost (i.e., too low a temperature) is the agent 

 directly concerned, but it accomplishes the injury because the shoots 

 are too succulent, and the tissues too feebly developed, to resist a 

 temperature which they would be perfectly able to resist if more 

 carbohydrates had been formed under a brighter light, and if less 

 water containing more oxygen had ascended their stems, and so 

 forth. 



It is at least difficult to class such cases, and they arise every day. 

 Who would have suspected that one result of bringing the larch down 

 from its mountain home would be to render it more liable to injury 

 from certain pests kept in abeyance on its native Alps, because it is 

 stimulated to put forth its young leaves when the insects are about, 

 which puncture the cortex and afford means of entrance .for certain 

 parasitic fungi ? 



On turning attention to the diseases referred to the action of living 

 organisms, we meet with difficulties rather greater than less, and it is- 

 chiefly on account of these that so many wild hypotheses are current 

 as to this class of diseases. 



Omitting more than a mere reference to the diseased or weakened 

 conditions due to competition with weeds, and with overbearing 

 associates, such as ThelepJiora laciniata, which may overshadow 

 young Conifers, and eventually kill them simply by depriving them of 

 lignt,* and to the various parasitic Phanerogams such as Loranihus,. 

 the mistleto, dodder, &c.,f we come to an enormous series of diseases 

 due to parasitic insects (and other animals) and fungi. 



The chief difficulty connected with the investigation of diseases 

 induced by fungi is due to the double set of complications involved. It 

 is difficult enough to unravel the tangled skeins of causes and effects 



* It. Hartig in ' Untersuchungen aus d. Eorstbot. Institut zu Mimchen,' 1880, 

 p. 164.. 



f See Solms-Laubach, " Ueber den Bau und die Entwicklung der Ernahrungs- 

 organe paras. Phaner." (' Pringsheim's Jahrbiicher,' vol. 6, 1867-8, p. 509). 



