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EXPLANATION OF PLATES XXIV-XXIX 
The following abbreviations are used in labeling the figures: st = sieve tube, 
sp sieve plate, 1 = lattice or lateral sieve field, cc = companion cell, p = paren- 
chyma, c = cambium, f = fibers, m = medullary ray. All figures are photographs 
of phloem except when otherwise designated. 
Pinus Strohus 
Fig. I. Radial section showing distribution of sieve plates upon the radial 
walls of the sieve tubes. X 250. 
Populus deltoides 
Fig. 2. End wall of sieve tube showing perforations in the sieve plates. X 450. 
Fig. 3. Side wall of sieve tube showing the lattice. The pores between the 
scalariform bars are very minute as compared with those in the sieve plates on the 
end wall. X 450. 
Fig. 4. Transverse section, one-year-old root showing the small size and 
number of sieve tubes as compared with the mature condition (fig. 7). X 100. 
Fig. 5. • Transverse section, three-year-old stem showing the same features 
as figure 4. X 100. 
Fig. 6. Transverse section, four-year stem showing same features as figure 4. 
X 100. 
Populus deltoides 
Fig. 7. Transverse section, mature root. Note increase in size and number 
of sieve tubes in series (figs. 4-7). X 100. 
Salix nigra 
Fig. 8. Tangential section, mature root, showing end walls of sieve tubes with 
sieve plates in section. The phloem of Salix and that of Populus are practically 
identical. X 100. 
