Secondary growth and occurrence of laticifers in the root of papaya (Carica papaya L.)

The pattern of secondary growth and occurrence of laticifers was studied in the tap root of papaya (Carica papaya L., Caricaceae). The vascular cambium was differentiated, producing secondary phloem centrifugally and xylem centripetally. Phloem was composed of sieve tube members, companion cells, fibres and parenchyma, whereas the xylem was mainly thin-walled parenchyma tissue with lignified vessels embedded in it. The root vascular tissue was characterized by the occurrence of isolated, narrow, but thick-walled articulated laticiferous cells. Fusiform cambial cells produced laticiferous cells only in the centripetal direction, whereas ray cambial cells produced these cells both centripetally and centrifugally. Laticifers underwent intrusive tip growth and their maturation was marked by the disappearance of transverse septa. These cells appeared as narrow segmented cells close to the cambial zone whereas in the xylem they were isolated and elongated with sinuous or constricted walls. Radial growth, structure and distribution of laticifers in the root are discussed.