PRESIDENTIAL ADDRESS SECTION C. 71 



psammosere). There is room for much further intensive study and 

 comparison of these species. Those that creep through the sand 

 are replaced by those that creep over the sand, and these in turn 

 by erect-growing shrubby species. Hydrophylax carnosa, Scaevola 

 thunbergii, Cyperus natalensis, etc., creep through the sand and 

 help to fix it. Ipomaea biloba grows partly through the sand and 

 partly over it. M ese mbrianthemum edule covers the surface over 

 wide patches near the bush., and various grasses (Stenotaphrum 

 glabrum, Dactyloctenium aegyptiacum, Sporobolus pungens) also 

 form large colonies or associes. The erect-growing, shrubby or 

 herbaceous species are very numerous, and form a transitional zone 

 to the psammophilous scrub and bush of the fixed line of sand 

 dunes. 



2. — Lagoon Vegetation. 



On the mud flats at the "drowned" river mouths, where the 

 water is brackish, the most important pioneers are species of 

 Salicornia (S. herbacea. S. natalensis) and Chenolea diffusa, but 

 this succession requires further analysis also. The dominant specie? 

 are the mangroves, which will be dealt with later. 



3. — Lake, Vlei, and Streambank Vegetation. 

 This type extends from the coast to the Drakensberg, and, 

 like other primitive stages of the succession, it varies only slightly 

 with rising altitude. The succession is very easily followed since 

 the various stages usually form distinct zones parallel to the water 

 edge. There is a certain similarity in this hydrosere in all parts 

 of the world. The stages in Natal are as follows: — 



A. Submerged Aquatics. Potamogeton spp., Hydrostachys 

 natalensis, Ceraphyllum demersnm, etc. 



B. Floating Aquatics. Species of Aponogeton, Nymphaea, 

 U tricnlaria , Lemna, Pistia, etc. 



C. Reed Zone. Phragmites often mixed with Typha. 



D. Semi-aquatics. A great variety of smaller herbaceous 

 species, few of which assume complete dominance. 



E. Cyperus — Mariscus Zone. Taller Cyperaceae. 



F. Vlei grasses. Species of Setaria, Pennisetum, Erianthus, 

 Leersia hexandra, and many others. 



G. Other vlei consocies or associes of herbs, shrubs or trees 

 which are transitional to sub-climax types. 



4. — Ruderal Vegetation. 



This represents the initial stages of secondary successions or 

 subseres, and consists of the weeds, whether herbaceous or shrubby, 

 which occur along roadsides and kafir pathways, in ditches and 

 quarries, on rubbish heaps, waste land and cultivated fields, or 

 where forest has been burnt out or cut down. The original vege- 

 tation has been destroyed and the soil disturbed, but as soon as 

 it is left alone the processes of repair are initiated and a new suc- 

 cession commences which leads up to the original grassveld, scrub, 



