RHIZOSHEATH OCCURRENCE IN SOUTH-AFRICAN GRASSES

Rhizosheaths are the sandy coatings which cover the entire length of e ach root of many of the indigenous grass species growing in South Afri ca. The results of an extensive herbarium survey showed that rhizoshea ths occur on more than 80% of the grass species studied, irrespective of the environmental conditions to which the individuals are exposed. Only 23 species did not have any sheath occurrence. The herbarium surv ey, together with growth experiments using Anthephora pubescens Nees, Digitaria eriantha Steud and Eragrostis pallens Hack, revealed that th e extent of the rhizosheaths (the thickness and consolidation of the s heaths) varies not only between but also within species. The within-sp ecies variation is a function of soil texture. The higher the sand con tent in the soil the greater the number of epidermal hairs produced an d the greater the extent of the sheaths. A. pubescens, D. eriantha and E. pallens individuals in soil with 80% sand had 75, 11 and 100 root hairs per centimetre of root length respectively. In comparison, the i ndividuals in soil with only 30% sand had 55, 5 and 45 root hairs per cm of root length respectively. This relationship indicates that while species have a genetic predisposition to sheath development, the exte nt to which they develop is a facultative response to soil texture.