Notes. 
181 
further progressive lignification of the phloem, especially as the 
lignin-reaction was often slight, or restricted to one side of an element 
or graduated in adjacent elements. It may be mentioned here that 
cambial activity had entirely ceased in this region of the stem. Where 
unlignified phloem was present, there was no meristematic layer 
between it and the xylem, but sieve-tubes and companion-cells were 
in contact with lignified xylem-elements. 
In the sieve-tubes and other tissue-elements referred to, the ligni- 
fication was determined by the use of phloroglucin, aniline sulphate, 
Molisch's 1 thymol-solution, and the chlorine and sodium-sulphite 
treatment. The staining of the elements with these different reagents 
corresponded with that of the xylem-elements of the same plant. 
The slight thickening and the lignification of the parenchymatous 
parts of the pericycle and of the medullary rays unite the primary 
(pericyclic) sclerenchyma-strands into a more definite mechanical 
system, attached to the strong xylem ring by the medullary rays. 
This must give greater rigidity, which no doubt is required by the 
heavy, fruiting capitula borne by the plant. 
Lignification of sieve-tubes is probably of rather rare occurrence 
among the Dicotyledons. Schumann 2 mentions that sclerosis of 
sieve-tubes and companion-cells takes place in some few Compositae, 
and gives Scorzonera hispanica and Aster thyrsiflorus as examples. 
He states that especially the sieve-plates are lignified, but with regard 
to reactions only mentions that they are stained yellow by iodine. 
Sclerification of sieve-tubes with frequent obliteration of the cavities 
is described by Vesque 3 in Betula alba , and by Boubier 4 in the same 
plant and in Alnus. Betula alba was consequently examined for 
comparison with Helianthus. In this plant numerous masses of 
sclerotic cells are present in the older secondary phloem, but in the 
specimen examined no obliteration of the cavities of sieve-tubes, nor 
lignification of their walls, was to be seen. Only a few sieve-plates 
were slightly lignified. The lignified elements of the phloem consist 
1 Molisch, Ein neues Coniferinreagens. Berichte der deutsch. bot. Ges., IV, 
1886, p. 303. 
2 Schumann, Beitr. z. Anat. d. Compositenstengels. Botan. Centralblatt, XLI 
(1890, 1), p. 195. 
3 Vesque, Anatomie compare de 1 ’ecorce. Annales des Sci. Nat., Bot, 6 e s^r., 
tom. ii, 1875, p. 178. 
4 Boubier, Anatomie systematique des Betulacees-Corylacees. Univ. de Geneve, ' 
Laborat. de Botan., 3® ser., fasc. vi, 1896, p. 58. 
