Observations on the formation, development, and structure of the tuber of Testudinaria elephantipes, and on the origin of the vegetative shoot

Abstract

SummaryThe young plants used for these observations were grown from seed collected in the Southern Karroo.The tuber of Testudinaria elephantipes is at first small, white, and spherical. Later it turns brown and develops a flattened or concave base and a thin continuous periderm.In the older, massive, semi-globular condition the tuber has a thick, deeply fissured, corky crust. Towards the base of the tuber the cork forms a thinner layer, broken only by the protrusion of roots. Various developmental stages are figured.There is no cork over a small area at the apex of the tuber. The growing point of the vegetative shoot is in this region, and is enclosed within a number of persistent scale leaves.The seed contains a small embryo embedded in considerable endosperm. There is no suggestion of tuberisation in the embryo.The development of the seedling is described and figured. A longitudinal section suggests that the plumular axis consists at first of little more than a stem apex and one leaf.Lack of opportunity for translocation of food synthesised is probably the cause of tuberisation. An explanation of the lack of symmetry is suggested. Tuberisation is not due to fungal infection.The anatomy of seedlings at different stages of development is described. The initial thickening of the hypocotyl results partly from cell division and partly from hypertrophy of the cortical parenchyma cells. Tuberisation is finally due to secondary growth originating in the pericycle.The early development of the cork crust and the growth zone is described.The structure of a three-year old tuber is dealt with in some detail. In general, the growth zone is hemispherical. Beneath the apex and just above the base no continuous meristem appears, though local growth zones may be found over the base. Vascular strands and medullary parenchyma are cut off towards the inside of the meristem, and secondary parenchyma towards the outside.The vascular tissue forms a small flat network under the apex and a larger network over the base, and these plates are connected by a complex system of anastomosing vascular strands running down the side of the tuber towards the inside of the growth zone. These vascular systems form growth rings. The bulk of the tissue cut off from the growth zone is parenchymatous.Fissures abutting on the medullary rays appear in the cork. The exposed parenchyma within the cork becomes suberised and a new band of cork is formed. Over the base of the tuber the original cork layer persists.An explanation is given for the origin of adventitious shoots.The sympodial origin of the seasonal vegetative shoots formed at the apex of the tuber is described and figured.The tuber is held to consist of a secondarily thickened hypocotyl and a very short epicotyl.