478 PROCEEDINGS OF THE ACADEMY OF [^lay, 



l)liigged tracheitis are confiued almost entirely to the summer wood, 

 aud occur iu largest number in the first six or eight years of 

 growth. Very few are found iu the annual rings subsequent to the 

 ninth. 



Older stem cankers show these plugged tracheids more plentifully 

 from the first annual ring of wood laid down to the last one (figs. 

 16, 17). The larger excrescences have them in greatest number. 

 The smaller excrescences are without them. At the point, how- 

 ever, where the tissues of the swellings join the main portion of 

 the stem, these plugged tracheids are so numeroas that they form 

 well-marked concentric bands. The following statement roughly 

 expresses the position and abundance of the plugged tracheids in 

 the several annual rings of a swelling with well-marked excres- 

 cences. 



Second Year Wood. Summer. Abundant, forming a compact 

 circular zone. 



Third Year Wood. Spring. Al)undaut, forming a loose zone. 



Fourth Year Wood. Summer. Sparse, except one area where 

 the plugged tracheids are closely aggregated together. At this 

 point the ring is much narrower, taking a U-shaped bend inw^ard. 

 The autumn wood here is wanting. The fourth annual ring is, 

 therefore, continuous except at this one point, where it becomes 

 bent inward. Instead of the fifth year wood following this in- 

 dented portion of the fourth year wood, it fills up the mdented area 

 with additional tracheids, so that at the end of the fifth year the 

 course of the autumn wood again conforms to a circular outline. 



Fifth Year Wood. Autumn. Sparse. 



Si.vth Year Wood. Late summer. Plenliful. 



Seventh Year Wood. Summer. Autumn. A few plugged 

 tracheids are found iu the summer wood. More are met with in 

 the autumn wood. 



Eighth Year Wood. Late spring. The plugged tracheids are 

 very abundant and in a definite circular zone. — Summer. Abun- 

 dant in two circular zones. Late summer. Scattered. 



Ninth Year Wood. Summer. Veiy few, but the plugged 

 tracheids apparently become brown lines coterminous wilh the 

 medullary rays. 



Tenth Year Wood. Autumn. Abundant and occupving in a 

 circular zone the region of the autuiim wood. The same brown 

 lines are traceable runniufr out to the cortex. 



