3^4 
L. H. MACDANIELS 
parenchyma, strongly suggests the possibihty that in seedHngs the 
conduction of protein material may take place, in large part, through 
the heavily pitted parenchyma. Of course it cannot be stated with 
certainty that such is the case, for it cannot be assumed that the few 
sieve tubes present in seedlings are not sufficient to supply the neces- 
sary conduction. Yet it seems reasonable to suppose that in a thrifty 
seedling considerable protein must be conducted through the phloem 
in excess of the apparent accommodation for such conduction through 
sieve tubes. It is well known that certain proteins may be conducted 
through parenchyma, as in the central cylinder of some of the mosses, 
the endings of the vascular bundles in leaves, and in the leptome paren- 
chyma, discussed by Haberlandt (4, p. 329). The structure of seedling 
phloem suggests that such may be the case there also, and that the 
sieve tube, being a highly complex and specialized structure, is not 
well developed until later in the ontogeny of the plant. 
In comparing the phloem of roots with that of stems, no essential 
differences are found in type of sieve tubes. This fact has been pre- 
viously brought out by Russow (16, p. 205). The phloem of the root 
shows minor differences from that of the stem, in that in the latter 
there is usually a greater amount of sclerenchyma. In the material 
examined, the only exception to this condition is the root of Viburnum 
Lentago, where the masses of sclerenchyma in the root were consider- 
ably larger than those in the stem. Another point to be briopght out 
is that the phloem of roots, particularly that of seedlings, shows a 
greater abundance of storage parenchyma differing from ordinary 
phloem parenchyma in that the cells are more nearly globose. In some 
seedlings practically all the phloem tissue is composed of cells of this 
sort. This condition may be so extreme as to give the impression that 
sieve tubes are fewer in number in seedling roots than in stems of the 
same age. Such seems to be the fact in a number of species, but as 
the observations lack the confirmation of actual count and measure- 
ment, it cannot be so stated with certainty. 
The great variation in the type and distribution of sclerenchyma 
in phloem tissue, a condition of course comparatively well known, is 
clearly brought out in this study. All conditions may exist from a 
nearly complete absence of sclerenchyma, as in Madura and Rhus, to 
practically solid masses of fibers and stone cells making up the entire 
older phloem of Fagus and Platanus. In the latter type all the phloem 
tissue, except that actually functioning, soon becomes converted into, 
