42 Dr. W. C. Williamson on 



feature consists in the remarkable number of equal 

 dichotomies which exist amongst these young twigs. In the 

 first eleven sections recorded in Table II. the very distinct 

 cylinders of Primary Xylem are composed of varying and 

 increasing numbers of tracheids, from 40 in C.N. 480 to 

 80 in C.N. 468 and 478. Beyond this stage of growth we 

 commence with C.N. 466 with a Primary Xylem cylinder 

 enlarged from -24 mm. in 480 to -4, and containing 105 

 tracheids. From this stage we advance to 345 tracheids in 

 C.N. 482 ; to 400 in C.N. 470 ; to 3,049 in C.N. 489, in which 

 the Primary Xylem has reached a mean diameter of 1 3*5 mm., 

 and 3,720 in C.N., 502, and with a Primary Xylem cylinder 

 with a diameter of 155 mm. In the latter case the cylinder 

 of Secondary Xylem has attained to a diameter of 35 mm., 

 whilst the medulla has advanced from a scarcely recognisable 

 feature in the youngest twigs to 11 mm. in C.N. 489, and 

 10mm. in 502. In this type we also have the two forms of 

 equal and unequal dichotomy. In each of C.N. 484 and 486 

 the two divisions of the Primary Xylem cylinder have 

 recovered their circular form, but in C.N. 503 we have 

 the condition represented in Memoir III., Fig. 20. In this 

 specimen both the two separated crescents remain unclosed, 

 shewing that the section has been made at a point 

 low down in the dichotomising branch. It now appears 

 certain that at this point the tracheal crescents never 

 became closed to form perfect cylinders. Between their 

 disunited horns the cells of the cortex and those of the 

 medulla met and blended in a continuous tissue. But in 

 the specimen 503 a further remarkable circumstance has 

 occurred. Each crescent of the Primary Xylem has become 

 invested by a crescent of Secondary Xylem, which has a 

 maximum thickness of 5 mm. On reaching the horns of 

 each of the Primary Xylem crescents, this secondary tissue 

 has grown closely round the points of the primary one and 

 pushed its way into the interiors of their contained 

 medullae. (See Memoir III., Fig. 20.) 



