372 
L. H. MACDANIELS 
Study of the sieve tube either adds to, or subtracts from, the evidence 
that herbaceous plants are highly advanced in the evolutionary scale. 
Another statement by the last-mentioned author (6), which is not 
confirmed by the present research, is to the effect that companion cells 
are very rare, if not wanting, in many of the lower woody dicotyledons. 
It is, of course, realized that the companion cell is often difficult of 
certain recognition, even in the best preparations. However, if small 
nucleated cells containing neither starch nor "tannin" are present in 
the corners or along the sides of the sieve tubes, giving the appearance 
of having been cut off from the sieve tubes by division, it seems reason- 
able to conclude that such cells are companion cells. Cells of this 
type were found in fair numbers in all the material studied except in a 
few species in which the sieve tubes were so badly crushed that com- 
panion cells could not be recognized with certainty The presence of 
companion cells in woody plants belonging to the lower families is 
shown quite plainly in the photomicrographs of the transverse sections 
of Populus deltoides (fig. 7), Juglatis nigra (fig. 13), and Liriodendron 
Julipifera (fig. 22). Further, the presence of companion cells in 
Salix and Populus is described by Strasburger (17, pp. 211, 214), who 
also states that in Salix the companion cell may extend across the entire 
radial width of the sieve tube. It is evident, however, that these cells 
do not occur in the lower woody dicotyledons in such abundance as in 
the more highly placed families, as shown in Tilia americana (fig. 39), 
Cephalanthus occidentalis (fig. 40), and Samhucus canadensis (fig. 43). 
On looking for correlation of sieve-tube type with type of vessel 
and wood parenchyma distribution, as indicated in table 2, it becomes 
evident that from the study of so small a number of species no marked 
relationships can be said to exist. For example, in the lower Amentif- 
erae the lowest type of sieve tube is present in the same plants with 
the porous vessel and, in the case of Salix nigra, with the vasicentric 
type of parenchyma. In the genus Pyrus, also, sieve tubes of type i 
are associated with porous vessels and diffuse parenchyma. Examples 
of the association of the high type of sieve tube with the scalariform 
type of vessel are not so numerous, Cornus paniculata being the only 
good example noted. 
In fact, although correlation between these structures does not 
stand out, it may be said that a fair percentage of cases shows sieve 
tubes, vessels, and parenchyma of approximately the same level of 
development according to types made. The large number of cases 
