698 JOURNAL OF FORESTRY 



hypertrophy with descriptions supporting this point — Fig. 28, "Blight 

 canker on branch showing characteristic swelHng and cracking of 

 bark on young luood," and Fig. 29, "The figure at the right shows a 

 young infection li^ith an enlargement, which is often characteristic in 

 vigorous branches." Heald" says "Enlarged lesions are apparently 

 the most frequent on vigorous shoots," and note also, the figures : 

 Figures 5 and 6 show "characteristic hypertrophy of two-year-old 

 shoot" ; Fig. 7 shows characteristic hypertrophy of vigorous shoot." 

 Other examples could, perhaps, be given, but the foregoing substanti- 

 ate, if they do not prove, the important fact in connection with the 

 present hypothesis^ — that hypertrophy, caused by a struggle between 

 the host and parasite resulting (if the host is victor) in a higher degree 

 of resistance, would naturally occur on the most vigorous parts of the 

 plant, i.e., young, rapidly growing sprouts. The writer's own obser- 

 vations the past field season in Pennsylvania and New Jersey are in 

 accord with the foregoing. 



Opportunity for Immunity 



To develop immunity requires time to accumulate resistance and 

 take advantage of it. Usually the organism succumbs to the invader 

 before it can increase resistance sufiicient to overcome the parasite. 

 The chestnut trees are killed, but the sprouts continue the life of the 

 individual unit (i.e., the coppice group) for a time. This period gives 

 a chance to accumulate resistance in the same individual, and has, in 

 fact, healed cankers. Other points may be attacked on the same stem 

 later and may not be so successful, but the fact that some cankers are 

 healed indicates that resistance is growing. It might be well here to 

 distinguish two kinds of resistance — one inherent in the plant, which 

 might be called static resistance, and the other, increasing or decreasing 

 as the struggle goes forward, could be called dynamic resistance. It is 

 the latter which the writer considers the more important as it holds 

 out the greatest hope. The former doubtless exists and through it, 

 by means of crossing foreign resistant strains, investigators expect to 

 develop high resistance and perhaps immunity. 



In order for immunity to develop, certain other requisites (besides 

 time) are necessary. In the first place, the invader or parasite must' 

 encounter the life force of the organism attacked; it must attack living^ 



"^ The Symptoms of Chestnut Blight, etc., Pennsylvania Chestnut Tree Blight 

 Commission, Bui. 5, at page 5. 



