6 3 2 



ZONES AND REGIONS [Pt. Ill, Sect. II 



Fig. 368. Flora of the South-west African desert: 

 Griqualand. Harpagophytum pinnatifidum, 

 Engl., a pedalineous South African tuberous 

 plant. After Engler. 



organs that are produced subsequent to 

 the cotyledons ; as the plant apparently 

 may attain an age of 100 years, these 

 leaves would seem to surpass by far all 

 other similar plant-members in duration 

 of life ; but this longevity is only mor- 

 phological, not physiological. The leaves 

 in fact are constantly renewed at their 

 bases by cell-formation and by growth, 

 whilst their apical parts gradually die and 

 dry up. Entire in young plants, the 

 margins of leaves of older plants are split 

 into shreds which lie in disorder on the 

 ground in many irregular folds. The 

 cone-like unisexual inflorescences spring 

 from pits in the axils of the leaves of the 

 monoecious plant. 



Inasmuch as allies comparable with 

 Welwitschia are completely wanting in 

 other localities — the remaining gnetace- 

 ous genera assume entirely different 

 forms— it were fruitless to inquire into 

 the part played by climatic conditions 

 in inducing this remarkable differentia- 

 tion of form. At any rate its unique 

 structure and mode of development 

 appears to be completely in accord with 

 the desert environment. 



The naras, Acanthosicyos horrida, is 

 systematically far less isolated, so that 

 the distinctions between its vegetative 

 organs and those of other Cucurbitaceae 

 may, to a great extent, be regarded as 

 adaptations to the climate. Whilst the 

 Welwitschia is confined to stony ground 

 between the sand-dunes, the shrubby 

 naras-bushes, up to one meter and 

 a half in height, clothe the summits 

 and slopes of the dunes. The copiously 

 branched, very rigid, green axes, which 

 are up to two centimeters in thickness, 

 bear strong opposite thorns in the axils 

 of rudimentary leaves. The roots, as in 

 most other desert plants, far exceed the 

 shoots, and are as thick as a man's arm, 

 and frequently more than fifteen meters 



