472 PROCEEDINGS OF THE ACADEMY OF [May, 



iu eoulour aud their walls are thicker thau those of subsequent 

 years. 



The phloem of a forly-oue-year-old stem differs materially from 

 that of a twenty-one-year-old stem in the absence of strongly 

 marked layers of bast fibres. Bast fibres are present, but they are 

 not in continuous layers. Even with the high power, it is difficult 

 to trace their conliuuity. The resin canals occupy a definite area, 

 viz., midway in the phloem. The medullary rays of the phloem 

 are continuous with those of the xylem, running out to the active 

 cork cambium (fig. 12). True cortical parenchyma is entirely 

 absent from such stems. The cork is in several well-marked zones 

 or bands. These bands are separated from each other by the sev- 

 eral layers of cork cambium. Seven well-marked layers of cork 

 cambium are met with in this forty-one-year-old stem. The outer 

 one is about to be pushed off by the sixth layer of cork. The 

 innermost cork cambium lies against the outer rows of phloem cells. 

 Several old fuuctionless resin canals are included in the mass of 

 dead exfoliating bark. They are conspicuous as rounded or ellipti- 

 cal holes in the mass of cork cells. The external appearance of 

 the bark of large trees is smooth than otherwise. It is usually from 

 three-quarters of an inch to nearly an inch in thickness, light 

 reddish-brown, and divided irregularly into narrow, flat connected 

 ridges, which are often spirally twisted around the stem, and separ- 

 ated into elongated loose or closely appressed plate-like fibrous 



Pathological Considerations. 



The pathological conditions which are followed by the striking 

 increase iu the size of the stems are of long duration, as evidenced 

 by a microscopic examination of the swellings. The fight for 

 supi'emacy, if such it may be called, is a long-drawn-out one. A 

 section was made across the smallest stem canker which com2)letely 

 girdled the whole stem. It was found upon examining the abnor- 

 mality that the same annual ring is not of a uniform thickness 

 throughout, being thinner in some parts than in others. The rings 

 are clearly much thicker than those of the normal stem. Sixteen 

 rings in all were counted in the region of the canker. This in- 

 creased width of the rings is due to an increase in the number of 

 tracheids produced by the cambium. These tracheids, as well as 



