Notes. 
320 
any considerable size. These two plants were left growing until December 12 , their 
upper leaves, which had remained green, having been killed by a frost a few days 
previously. The other plants were collected at different times from October 15 to 
December 6, in each case after the fruiting of the main flower-heads had been 
completed. 
The proportional amount of lignified tissue was found to vary greatly in 
different plants, and in different parts of the same plant. In most cases lignification 
of the phloem is more general in the upper part of the stem than near the base 
(e.g. two inches above the level of the soil), the middle region being intermediate. 
The pericycle behaves similarly, the whole or the greater part of this tissue being 
lignified in the upper region of the stem, while at the base usually very little of it 
had undergone lignification, or sometimes none. 
In the plant showing the greatest amount of lignification, there were, at intervals, 
small groups of elements having cellulose walls and consisting of two or three sieve- 
tubes with companion-cells, while opposite the primary masses of sclerenchyma 
similar but rather larger groups were present ; apart from these the whole of the 
phloem and pericycle had become lignified. No cambium remained, so that 
a transverse section showed a continuous mass of lignified tissue, comprising the 
xylem and the tissues between the latter and the endodermis \ and interrupted only 
by small islands of unaltered phloem. 
Lignification of the phloem generally begins immediately on the inner side of 
the primary groups of sclerenchyma, and proceeds from these towards the xylem. 
Usually the medullary rays in the phloem undergo gradual lignification from within 
(i.e. from the limit of the xylem) outwards, and this often begins a little later than in 
the first phloem-elements. The groups of sclerenchyma may become connected 
with the xylem by bridges of lignified tissue at a time when there is very little 
lignified tissue at intermediate points. As a rule lignification of the pericycle begins, 
often comparatively late, at the sides of the groups of sclerenchyma, and proceeds 
tangentially. Hence it appears that in all these tissues lignification generally takes 
place in elements adjacent to previously lignified tissue. 
From the data obtained it was not found possible to deduce the cause of 
lignification of the phloem. The largest plant (stem eight feet high, and 3*8 cm. in 
diameter at the base) showed the greatest amount of lignification in the basal region, 
and the smallest plant (stem three feet high, and less than 1 cm. in diameter at the 
base) showed the least amount, but the remaining plants gave no sort of proportion 
between the degree of lignification and the size of the stem. Neither was there 
a regular relation to the age of the plants. Again the two plants, from which the 
flower-buds were cut off, showed less lignification at the base of the stem than the 
majority of the other plants, but the distinction was not sufficient to warrant any 
definite conclusion. Possibly lignification of the walls may set in near the time 
of the death of the elements, and this may depend on several factors affecting the 
supply and demand of food-substances. 
Lignification of the proteid contents of sieve-tubes was seen in the roots of all 
1 Lignification appears to stop accurately at the endodermis ; no cases of lignified cortical cells 
were observed. 
