ECCENTRIC SECONDARY GROWTH IN CORDYLINE AND OTHER AGAVACEAE (MONOCOTYLEDONAE) AND ITS CORRELATION WITH AUXIN DISTRIBUTION

Eccentric secondary growth is described and illustrated in detail for the first time in horizontal stems of Cordyline, Dracaena, Yucca and Beaucarnea (= Nolina) with up to 13 times more secondary tissues on the lower side than on the upper side. In Cordyline the secondary tissues on the lower side are rhizome-like in having less lignification of ground parenchyma and more diffuse and smaller secondary bundles than in the vertical stem. In Cordyline, Yucca, Beaucarnea, and D. reflexa the ground parenchyma cells are larger on the lower side. The vascular bundles are significantly larger on the lower sides of Beaucarnea and D. reflexa and are smaller on the lower side of D. fragrans. The occurrence of growth rings and eccentric growth is related to changes in cambial activity. There is close correspondence between enhanced cambial activity and high auxin levels on the lower side which have been reported elsewhere. However, there is no evidence of reaction wood tracheids on either the upper or lower sides.