LIGNIFICATION OF MATURE PHLOEM IN HERBACEOUS TYPES

Abstract

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 (Horngewebe) was applied by Wigand (I863) to such a mass of crushed tissue. The principal facts concerning the transformations undergone by the phloem of woody angiosperms have been generally known since the time of this writer. Previous to this date both -Oudemans (I862) and Rauwenhoff (I869) appear to have described obliterated structures in the phloem, Oudemans in I855, and Rauwenhoff in I859. These two writers, together with Moeller (I875) and Tschirch (I889), 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 neighboring 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 (I884) indicated some doubt as to the extent to which the cambiform cells of the phloem are affected in obliteration, although he described the crilshing of the sieve tubes. Bliesnick (I89I) 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. Lignification 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