270 PSAMMOPHYTES sect, x 



partly to Nitraria tridentata. In German south-west Africa there are, 

 here and there, impenetrable bushlands clothing the dune-hillocks of 

 the shore with plant-types which either assume the Erica-, myrtle-, or 

 oleander-form, but belong to entirely different families, or are thickly 

 coated with woolly hairs, or are fitted in some other way to avoid rapid 

 transpiration. One very remarkable shrub in the African dunes is the 

 cucurbitaceous Acanthosicyos horrida which attains man's height. It 

 lacks leaves ; but paired thorns are crowded so densely and in such a 

 manner on the felted branches that impenetrable bush arises similar to 

 that formed in Europe by Hippophae. The roots may attain a length 

 of 15 metres or more, and the thickness of the human arm ; they descend 

 as far as the ground-water. Wind causes sand to accumulate round the 

 plants, whose shoots grow with the rising sand and repeatedly emerge 

 from this, just as is the case with the sea-marram in Europe.^ 



Asia. In regard to tracts of sand in Asia it may be added that on 

 sand-dunes in the Kirghiz Steppe, Pinus, Betula, Populus, Salix, and 

 Ulmus grow in company. The most efficient sand-fixing plants of the 

 Transcaspian steppes are Carex physodes, Aristida pungens, and a number 

 of shrubs. On sandy soil we find bluish-green and aphyllous species 

 of Calligonum, Ephedra, and of the papilionaceous Ammodendron, also 

 the remarkable saxaul-tree (Haloxylon Ammodendron) which almost 

 gives rise to forest .^ 



North America. In North America the phenomena of dune-production 

 in a forest country have been described by Cowles,^ in connexion with his 

 observations on dunes bounding the shores of Lake Michigan. Here 

 first rank as a dune-producer is taken by Psamma arenaria, and second 

 rank by Agropyron dasystachyum, Elymus canadensis, Calamagrostis 

 longifolia, Sahx adenophylla, S. glaucophylla, Prunus pumila, and Populus 

 monilifera. When one of the willow-shrubs is buried by the sand, roots 

 shoot forth from the buried branches. Populus monilifera and P. balsa- 

 mifera also germinate on the shore and sometimes give rise to groups of 

 trees sufficiently close to cause an accumulation of sand. As the dunes grow 

 the conditions of life become unfavourable to the dune-producing plants. 

 The dune becomes high and dry and the length of life of the plants is not 

 indefinite, so that they die and the sand is blown away. Even after death 

 Calamagrostis and Psamma can act as efficient sand-fixing agents, whereas 

 other species, such as those of Populus, cannot act in this manner. On 

 the lee side willows, poplars, Vitis cordifolia, grasses, and other dune- 

 plants grow, whereas on the windward side these cannot establish them- 

 selves, as not one of them is able to fix the rapidly shifting dunes. Not 

 until the distance from the shore is greater, the wind weaker, and protec- 

 tion is provided by other dunes, does the dune commence to be clothed 

 with vegetation. The first plants to appear on the lee side of the slowly 

 moving dunes are Psamma arenaria, then Asclepias Cornuti, Equisetum 

 hiemale, and Calamagrostis longifolia. In the course of a few years the 

 lee side becomes overgrown by the shrubs and trees, Cornus stolonifera, 

 Sahx adenophylla and S. glaucophylla, Vitis cordifolia, Prunus virginiana, 

 and Tilia americana. Shrubs suppress the herbs ; lime-trees grow and 



' Marloth, 1888. 



" See the Sections on Halophytes (Chapter LXVII), Deserts, and Steppes. 



' Cowles, 1899. 



