FIBROVASCULAR TISSUES: PHLOEM 109 



and more or less loaded with starch. These are the parenchyma 

 of the phloem. We are not acquainted with the origin of paren- 

 chymatous cells in the phloem for reasons indicated at the outset 

 of the present chapter. Farther inward lie the larger conductive 

 elements of the phloem, known as the metaphloem. These are 

 cells which have no contents but a delicate lining of protoplasm, 

 and this does not appear in the illustration. The elements under 

 discussion as well as those of the protophloem are known, on 

 account of certain structural features to be described later, as 

 sieve tubes. It is important to note in the present connection that 

 sieve tissues do not present the variety of modes of development 

 which characterizes the xylem. Almost invariably in this cate- 

 gory of tissues the first-formed elements are external and those of 

 later appearance are laid down in centripetal order. As a con- 

 sequence the terms exarch, mesarch, and endarch, which are so sig- 

 nificant in connection with the development of the primary wood, 

 have little bearing in the case of the sieve tissues. Internal to the 

 larger sieve elements of the metaphloem lies a more or less continu- 

 ous band of parenchyma, separating the phloem from the xylem. 

 The tracheary elements of the wood are individually more or less 

 completely surrounded with parenchyma. The phloem does not 

 form a continuous band about the xylem, but is interrupted at the 

 two ends of the elongated mass of wood. In certain ferns the tissues 

 of the phloem occur only on one side of the bundle, in which case the 

 strand of fibrovascular tissue is known as collateral. Where the 

 phloem forms a complete jacket about the xylem the condition 

 is known as concentric; and where, as in the bundle appearing in 

 the figure under discussion, the sieve tissues are confined to two 

 opposite sides the condition may be designated as bicollateral. 



In Fig. 85 is represented a longitudinal view of part of the 

 same bundle. The sieve tubes now appear as elongated cells 

 with tapering ends and sculptured walls. The relief of the walls 

 consists of somewhat angular crowded areas, although on the 

 whole they are tolerably evenly distributed. These areas are per- 

 forated with very small simple pits and are the sieve plates. It is 

 these structures which give to the sieve tube its name. After the 

 characteristic elements of the phloem have reached a certain age 



