Stem anatomy and development of inter- and intraxylary phloem in Leptadenia pyrotechnica (Forssk.) Decne. (Asclepiadaceae)

Modification of external morphology and internal structure of plants is a key feature of their successful survival in extreme habitats. They adapt to arid habitats not only by modifying their leaves, but also show several modifications in their conducting system. Therefore, the present study is aimed to investigate the pattern of secondary growth in Leptadenia pyrotechnica (Forssk.) Decne., (Asclepiadaceae), one such species growing in Kachchh district, an arid region of Gujarat State. A single ring of vascular cambium, responsible for radial growth, divided bidirectionally and formed the secondary xylem centripetally and the phloem centrifugally. After a short period of secondary xylem differentiation, small arcs of cambium began to form secondary phloem centripetally instead of secondary xylem. After a short duration of such secondary phloem formation, these segments of cambium resumed their normal function to produce secondary xylem internally. Thus, the phloem strands became embedded within the secondary xylem and formed interxylary phloem islands. Such a recurrent behavior of the vascular cambium resulted in the formation of several patches of interxylary phloem islands. In thick stems the earlier formed non-conducting interxylary phloem showed heavy accumulation of callose on the sieve plates followed by their crushing in response to the addition of new sieve elements. Development of intraxylary phloem is also observed from the cells situated on the pith margin. As secondary growth progresses further, small arcs of internal cambium get initiated between the protoxylem and intraxylary phloem. In the secondary xylem, some of the vessels are exceptionally thick-walled, which may be associated with dry habitats in order to protect the vessel from collapsing during the dryer part of the year. The inter- and intraxylary phloem may also be an adaptive feature to prevent the sieve elements to become non-conducting during summer when the temperature is much higher.