LIGNIFICATION OF MATURE PHLOEM IN HERBACEOUS TYPES 
Carl L. Wilson 
(Received for publication August 5, 192 1) 
Secondary growth in the woody stems of gymnosperms and angiosperms 
often results in the formation of annual rings in the xylem, but in the phloem 
no similar arrangement of elements is usually to be observed. The sieve 
tubes, which are generally the principal elements of the phloem, are crushed 
at the end of each period of growth by the pressure of surrounding cells and 
tissues. To these changes taking place in the secondary phloem the term 
obliteration has been applied for a long time. The alterations are initiated 
by the disappearance of the contents of the sieve tubes at the end of each 
growth period, soon followed by a crushing and distortion of the sieve tubes, 
and, in the case of angiosperms, of companion cells also. The sieve plates 
may be broken down, and the wall of the sieve tube takes on a swollen 
appearance. In an advanced condition of obliteration, the lumen of the 
sieve tube entirely disappears, being represented only by a line, and the 
whole mass of crushed sieve tubes takes on a horn-like consistency. During 
these changes the structure of the phloem parenchyma does not appear to 
be affected. 
The term horn prosenchym (Horngeivebe) was applied by Wigand (1863) 
to such a mass of crushed tissue. The principal facts concerning the trans- 
formations undergone by the phloem of woody angiosperms have been 
generally known since the time of this writer. Previous to this date both 
Oudemans (1862) and Rauwenhoff (1869) appear to have described oblit- 
erated structures in the phloem, Oudemans in 1855, and Rauwenhoff in 
1859. These two writers, together with Moeller (1875) and Tschirch 
(1889), showed the crushed tissue to be composed of obliterated sieve tubes. 
Tschirch pointed out that the sieve tubes, including cambiform cells and 
companion cells, having lost the principal portion of their contents at the end 
of the growing season, become pressed together through the turgor of neigh- 
boring cells, so that only a small, cleft-shaped lumen remains. He believed 
that a subsequent thickening of the wall does not take place. De Bary (1884) 
indicated some doubt as to the extent to which the cambiform cells of the 
phloem are affected in obliteration, although he described the crushing 
of the sieve tubes. Bliesnick (1891) found that the phloem parenchyma 
takes no part in obliteration. His conclusions are essentially the same 
as those of Tschirch with regard to the causes of obliteration. Lig- 
nification of sieve plates was found in Clematis, and in another case he 
mentions that he found complete lignification of the obliterated phloem 
elements. His results differ from those of previous investigators in that 
239 
