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HISTOLOGY OF PHLOEM IN WOODY ANGIOSPERMS 36 1 
Fraxinus nigra 
Mature. Sieve tubes of type 3, the single plate either transverse or somewhat 
oblique; pores in plate large; lattice a faintly developed network with fine 
pores. 
Companion cells not observed on account of crushed condition of sieve tubes. 
Parenchyma abundant; of divided-cambiform type, not very heavily pitted. 
Cephalanthus occidentalis 
Mature root (fig, 40). Sieve tubes long and narrow, with one or two round sieve 
plates either transverse or oblique; lattice not well developed. 
Companion cells abundant. 
Parenchyma, scattered cells of the conducting type, heavily pitted. 
Samhucus canadensis 
Mature (figs. 43, 44). Sieve tubes few, small, with oblique end walls, 10-20 plates; 
20-30 n in width, sometimes rather widely separated on the side walls; lattice 
not well developed. 
Companion cells present in most of sieve tubes. 
Parenchyma very abundant; short cambiform type, heavily pitted laterally. 
Three-year root (fig. 42). Sieve tubes small and few, of same type as mature; plates 
8-18 IX in width. 
Parenchyma full of starch, as are all cells of xylem except vessels; not con- 
spicuously pitted. 
Samhucus racemosa 
One-year stem (fig. 41). Sieve tubes small and scarce; 1-5 sieve plates on end wall. 
Companion cells present. 
From the foregoing descriptions of material it becomes evident 
that there are certain differences and certain similarities between the 
conditions found in the phloem of seedlings and in that of mature 
plants. Among the similarities, perhaps the most significant observa- 
tion, from the standpoint of the present research, is that apparently 
there is no fundamental difference between the type of sieve tube 
found in seedlings and that found in the mature condition. Species 
having sieve tubes with transverse sieve plates in the mature plant 
show the same type in the seedling, e. g., Robinia, Ailanthus, and 
Ulmus. In plants with sieve tubes of types i and 2, such as Populus, 
Juglans, Liriodendron, and Sambucus, the similarity between the 
sieve tubes of seedlings and adults is apparent, though not so marked, 
in that the seedlings of these plants show sieve tubes with a smaller 
number of sieve plates and frequently with a single transverse plate. 
Such a reduction in number of plates might perhaps be taken to indi- 
