HISTOLOGY OF PHLOEM IN WOODY ANGIO SPERMS 
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Sieve tubes:^ Size; abundance; distribution; type of end wall — i. e., type i, very 
oblique with 10-20 sieve plates separated by scalariform bars; type 2, with 
oblique end wall with 2-10 sieve plates between scalariform bars; and type 3, 
with a single plate, either transverse or slightly oblique; lattice-description 
(by lattice is meant the sieve fields on the side walls of the sieve tubes). 
Companion cells: abundance; distribution. 
Parenchyma: distribution; abundance; type, whether "cambiform" {i. e., 
elongated), prosenchymatous, or "conducting" (i. e, shorter, larger in 
diameter, and thin-walled, usually in series vertically. These terms are 
after Haberlandt, 4). 
Salix nigra 
Mature. Sieve tubes abundant, in irregular tangential rows between bands of 
fibers, not collapsed even in older portions; end walls oblique, of first type, 
with 8-15 sieve plates; lattice with fine pores between cellulose bands, almost 
identical with that of Populus deltoides (fig. 7); pores in sieve plates 2-3 fj. 
in diameter, those in sieve fields of lattice about 0.4-0.5 ix. 
Companion cells in the corners of about one third the sieve tubes, sometimes 
extending the entire width. 
Parenchyma of one type, the divided cambiform; for the most part filled with 
dark-staining granules; heavily pitted on end and radial walls with sieve-like 
pits; cells about equal to sieve tubes in number; in tangential bands along 
the bands of fibers and scattered among the sieve tubes. 
Growing. No new features except that fewer parenchyma cells are filled with 
tannin^ in the current year's growth. 
Mature root. Differs from mature stem only in having somewhat less sclerenchyma 
(fig. 8). 
Two-year stem from rapidly growing plant. Sieve tubes few in number and small as 
compared with mature conditions; most elements with oblique end walls, a 
few with transverse sieve plates; on oblique end walls the plates are more 
widely separated than in mature phloem; lattice not well developed; walls 
of tubes apparently much thinner than in mature sieve tubes; actual breadth 
of sieve plates 10-20 [x as contrasted with 20-50 n in mature. 
Companion cells present in normal numbers. 
Parenchyma of same type as mature. 
Populus deltoides 
Mature stem. Phloem identical with that of Salix nigra except in minor details. 
Smaller proportion of parenchyma filled with dark-staining granules than in 
Salix. 
2 Although in its best usage the term sieve tube refers to a series of cells joined 
end to end (see De Bary, 2), for convenience the term is here also used to designate 
a single element or cell unit of a sieve tube. 
3 In this paper the term tannin is used to signify the dark brown cell content 
that is abundant in many plants. In many cases it is chemically tannin, but in 
others it is organic material of similar appearance. 
