354 L. H. MACDANIELS 
Width of sieve plates 30-45 /z. Width of sieve pores 3.5-5.5 n (fig. 2). Pores on 
side walls mere dots not over 0.5-0.6 /x, not so distinct and clear cut as in 
sieve plate (fig. 3). 
Mature growing and mature root show no new features in phloem except reduction 
of sclerenchyma in latter (fig. 7). 
Fotir-year stem. Rapid growth (fig. 6). Sieve tubes abundant, same type as mature 
but smaller, width of sieve plates 15-25 ix; plates not so crowded on terminal 
wall as in mature; lattice not well developed. 
Companion cells in normal numbers. 
Parenchyma abundant, of two types: one short, wide, usually filled with tannin, 
the other elongate, heavily pitted on the radial walls and not containing 
tannin. 
Four-year root. Same as four-year stem except for increase in storage tissue and 
reduction of amount of sclerenchyma; all parenchyma full of starch. 
Three-year stem. Slow growth (fig. 5). Same general features as in four-year stem 
but number and size of sieve tubes greatly reduced; lattice not well developed; 
width of sieve plates 12-20 fx. 
Companion cells present. 
Tissue packed with starch; length of sieve tubes less than in mature. 
Three-year root. Sieve tubes smaller and fewer than in stem of same age. Pores 
on sieve plates not well developed; lattice not well developed, none observed; 
storage tissue abundant, packed with starch. Fewer sieve plates on end wall 
than in older material. 
One-year stem (fig. 4). Sieve tubes very small and scarce, much more so than in 
the three-year material, difficult to find; some sieve plates transverse, usually 
only two or three plates on oblique walls. Width of sieve plates 6-10 m; 
parenchyma full of starch. 
One-year root. Same in general as one-year stem. Sieve tubes fewer in proportion 
to parenchyma; some with transverse walls in same section with those with 
as many as six sieve plates on an oblique end wall. 
Juglans nigra 
Mature (fig. 15). Phloem very much like that of Salix and Populus in general 
features. Sieve tubes abundant, scattered among parenchyma, of lowest 
type, with very oblique end walls, sometimes having as many as 20 sieve 
plates; lattice well developed, much as in Carya (fig, 9) but not so regular; 
pores visible but not more than 0.5-0.6 fx in diameter; pores on sieve plates 
1.8 to 3.5 At, as in Populus (figs, 2, 3). 
Companion cells not abundant, probably present in about one sieve tube out of 
ten. 
Parenchyma of divided cambiform type, frequently filled with dark staining 
substance. 
N.S. 12 to N.S. 2. Phloem taken from the top at points at 12, 10, 8, 6, 4, and 2 
years of age showed gradual decrease in size of sieve plate from 25-40 ix in 
width to 12-20 IX in the two-year material; also a reduction in number of 
sieve tubes as compared with the number of parenchyma cells, a reduction 
very similar to that observed in a series of seedlings of different ages. 
