OF THE EASTERN UNITED STATES 365 



elements in the cortex are smaller and fewer than in the above ground 

 stem. The pericycle differs from that of the aerial stem in having widely 

 separated small groups of narrow sclerenchyma elements. 



The phloem region is composed of wider elements than exist in the 

 similar region of the above ground stem. It is traversed by broader 

 medullary-rays which are from 1-4 to 1-5, rarely 1-6, rows of cells wide 

 (primary medullary-rays) or from 1-2 to 1-3 rows of cells wide (secondary 

 medullary-rays). Bast fibres are present in both proto- and meta- 

 phloem regions, in small groups, which are surrounded by crystal fibres 

 containing monocUnic prisms of calcium oxalate. The pitted tracheae 

 of the xylem are fewer than in the aerial stem and about twice as broad. 

 The largest have a maximum breadth of 81.56 /x. The xylem medullary- 

 rays are numerous and have cells considerably broader than those of the 

 aerial stem. The primary medullary-rays are mostly 1-5, rarely 1-6, 

 rows of cells wide. The secondary medullary-rays are from 1-2 to 1-3 

 rows of cells in breadth. The autumn wood is similar to that of the 

 aerial stem. The pith region is uniformly wider, its cells somewhat 

 smaller, and with thinner walls than the corresponding area of the aerial 

 stem. iSIany of its cells, like those of the cortex, contain starch and 

 gummy lignin. 



Aerial stem of Myrica Macfarlanei, Youngken 

 (M. cerifera x M. Carolinensis) 

 The aerial stem of the hybrid resembles that of M. cerifera in the 

 following structural details: — (a) a continuous sclerenchyme zone in the 

 pericycle, (b) the tendency of the phloem masses to become arched in 

 their outer portions due to the broadening out of the medullary rays at 

 their extremities, (c) the presence of many bast fibres accompanied by 

 crystal fibres as well as stone cells in the phloem, (d) the uniformity 

 in distribution of the tracheae. It resembles the aerial stem of M. 

 Carolinensis in the size and relative number of tracheae found in the 

 protoxylem. 



It differs from both parents as follows — The autumnal wood is 

 intermedate in thickness; the pitted tracheae are fewer than in M. 

 cerifera, more numerous than in M. Carolinensis; the mean diameter of 

 the tracheae is intermediate between that of both parents (Plate 87, Fig. 

 20). The primary medullary rays are 1-3, rarely 1-4, rows of cells wide; 

 the secondary, 1-2 rows in width. Other stem structures common to 

 both parents are likewise seen in the hybrid. 



