Homoeogenetic induction

Homoeogenetic Induction. The experiments outlined above and many others repeatedly draw attention to the fact that differentiation that has already occurred can induce the development of new differentiation of the same kind. Thus, xylem induces the formation of new xylem in the callus, which then connects with the xylem already present (e.g. in Cichorium, Syringa, and Coleus) or orients itself with respect to it (Fig. 209). The phloem behaves similarly. This phenomenon is known as homoeogenetic induction. 

Now there are objections to this concept of homoeogenetic induction, which are illustrated by an experiment carried out by Torrey (Fig. 212). In the roots of peas (Pisum sativum) there is a triarchic vascular bundle. As is well-known the designation triarch, tetrarch, etc., is determined by the number of radially distributed xylem strands. If root segments are grown in culture, they first regenerate a new root tip. In the presence of lAA, however, a triarchic vascular bundle is no longer formed in the cells of these new root tips but a vascular bundle with a different number of... 

Thus the last word on homoeogenetic induction has still not been said. However, we can state with certainty that phytohormones not only play a role in stimulating division in an already existing cambium (pages 198, 202), they also participate in the differentiation of the cells generated by this cambium to form elements of the xylem and phloem. As usual, the interplay of several phytohormones among themselves and with other factors determines the kind of differentiation (xylem or phloem, etc.). 

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