Studies on the anomalous cambial activity in Doxantha unguis-cati (Bignoniaceae). I. Development of the vascular pattern

The development of the vascular pattern was studied in Doxantha unguis-cati as a prelude to correlating the vascular system to the anomalous cambial activity observed in the mature stem. The shoot apex of this liana has a two-layered mantle covering a corpus of a mother cell zone and a flanking meristem. Just below the shoot apex a cylinder of procambium differentiates. The first primary phloem can be recognized in the third internode and the first primary xylem in the fourth or fifth internodes. As the internodes elongate so do the procambial cells. The cells of the pith, on the other hand, are shorter than the procambial cells and accommodate to the extensive internode elongation by cell division. The vascular system of the young shoot includes not only bundles, but also individual strands of phloem. The stem has multilacunar nodes from which five bundles and two phloem strands depart into each of the oppositely paired leaves. Within the stem there are four continuous bundles to which all other bundles connect. The location of these four bundles corresponds to the formation of the characteristic anomalous cambium observed in the secondary growth of the older stem. The relationship between the vascular pattern of D. unguis-cati to other vascular patterns is also discussed.