May, 1922] 
WILSON — LIGNIFICATION OF MATURE PHLOEM 
241 
also become lignified. No conditions such as were described by Schumann 
and Boodle have been noted. The sieve tubes and companion cells have 
not been found to be lignified, and appear normal and functional in every 
way. In the transverse section of a stem shown in figure i, Plate XIII, 
(upper left), a phloem group is seen enclosed between the bundle cap and 
the xylem. All the elements exclusive of the sieve tubes and companion 
cells appear lignified with the exception of several groups just outside 
the cambium; these latter may be tissue which has not yet matured, or 
may consist of sieve tubes. In the transverse section shown in figure 2 
(upper right), the whole of the phloem is not shown, but the same condi- 
tion is seen to prvail. Some unlignified elements are seen just under the 
bundle cap; these are probably sieve tubes of the protophloem. Figure 3 
(lower left) is an enlarged view of a portion of the same region shown 
in figure 2, showing unlignified sieve tubes and companion cells surrounded 
by lignified parenchyma. Figure 4 (lower right) represents a longi- 
tudinal section through a region similar to that shown in the preceding 
figure. The walls of the sieve tubes (s) appear black, in contrast to the 
lighter color of the lignified parenchyma walls. Several cambiform cells 
with lignified walls are indicated at x, a nucleus and stored food material 
being clearly shown. 
Some correlation, at least, appears to exist between the presence of 
lignification in the stem and in the root. In an examination of the tap 
roots of several species possessing lignification, lignification of phloem 
parenchyma was found. The following plants were found to possess 
lignified phloem elements in both root and stem : 
As stated previously, no conditions comparable to those described by 
Boodle have been noted, either in Helianthus annuus or in other plants to 
which he referred. An examination of a large number of stems of H. annuus 
was made to find such conditions if possible. In one case only was a de- 
parture from the usual condition found. This was in a mature stem, about 
one half inch in diameter, which exhibited an unmistakable lignification of 
phloem parenchyma when examined by means of free-hand and microtome 
sections. 
Referring to the following list, several interesting facts may be brought 
out. It will be noted that all the forms in which lignification has been found 
belong to the first series of the Compositae, the TubuHflorae, no lignification 
having been found in any species falling within the Liguliflorae. Lignifica- 
tion does not seem to occur in all the species of a genus in which lignification 
has been found. Illustrations of this fact can be seen in nearly all the 
genera listed. It has also been found in a few cases, which could probably 
Solidago arguta Ait. 
S. rugosa Mill, 
5. serotina Ait. 
Aster ericoides L. 
A. unihellatus Mill. 
A. puniceus L. 
Solidago graminifolia (L.) Salisb. 
