HISTOLOGY OF PHLOEM IN WOODY ANGIOSPERMS 
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radial walls, most abundant on radial; side walls of younger tubes completely 
covered with a layer of bluish-staining substance, probably callus. Com- 
panion cells abundant in the corners and across the sides of the tubes. 
Parenchyma not abundant; of divided-cambiform type, heavily pitted among 
the radial walls. 
Mature root (figs. 22-25). Differs from mature stem only in having less sclerenchyma. 
Three-year stem. Phloem areas small, between large fan-shaped rays. Sieve tubes 
small and few as compared with amount of parenchyma; sieve plates trans- 
verse in majority of cases; lattice not apparent. 
Parenchyma of two types, the elongate phloem parenchyma distinct from the 
storage type. 
Three-year root. Differs from stem of same age only in having no fibers and in having 
smaller amount of phloem as compared with storage tissue. 
Twig. Sieve tubes small with sieve plates either on an oblique end wall or along 
sides of tubes. 
Parenchyma of same type as in mature. 
Sassafras variifolium 
Root of small tree. Sieve tubes not abundant; single sieve plates transverse or. 
slightly oblique; lattice not well developed. 
Companion cells not observed on account of collapsed condition of sieve tubes 
Parenchyma of divided-cambiform type; heavily pitted upon radial walls. 
Benzoin aestivale 
Mature. Sieve tubes small, nearly occluded with callus-like substance; sieve plates 
single, either transverse or oblique; lattice not conspicuous. 
Companion cells present in about one half of the sieve tubes. 
Parenchyma of divided-cambiform type, partially filled with tannin-like sub- 
stance; frequent large secretory cells. 
Platanus occidentalis 
Mature. Sieve tubes crushed except next cambium; large with oblique end walls; 
type 2, with 3-10 large plates 35-45 ix wide, frequently separated rather 
widely; lattice in the form of small pore areas upon both radial and tangential 
walls; no well-defined cross bars as in Populus. 
Companion cells present in about one third of sieve tubes. 
Parenchyma of divided-cambiform type, frequently filled with tannin; heavily 
pitted on the radial, and to some extent on the tangential walls. 
Four-year stem. Phloem small in amount, in restricted areas between rays; sieve 
tubes small with transverse end plates, or plates upon side wall; plates small, 
8-12 fx in width; lattice not apparent on uniformly thin side walls. 
Four-year root. Like stem of same age. 
Two-year stem. Sieve tubes showing sieve plates widely separated upon side walls ; 
width of plates 6-8 ju. 
Parenchyma elongated with abundant pitting. 
Two-year root. Like stem of same age. 
