[Phytochrome-mediated control of xylem differentiation in the hypocotyl of the mustard seedling (Sinapis alba L.)].

P730, the active phytochrome, causes an increased formation of xylem elements (tracheids and vessel elements) in the hypocotyl of the mustard seedling (Figs. 3,4). On the other hand, the pattern of differentiation of the bundles and the course of the bundles within the hypocotyl (Figs. 1,2) are the same in etiolated as well as in illuminated seedlings.-It has been concluded that in connection with bundle differentiation P730 acts only as a "trigger" at the level of "secondary differentiation". The pattern of differentiation is laid down in the course of "primary differentiation" which apparently is not influenced by P730. The same problem has been dealt with more in detail in a foregoing paper (WAGNER and MOHR, 1966b).