December 20, 1912] 



SCIENCE 



867 



plants of delicate appearance have tubers or tuber- 

 ous roots {Erodium Mrtum and Erodium arlior- 

 escens) sunk deep in the strong ground for the 

 storing of reserves of nutriment adequate to main- 

 tain them alive through long months of absolute 

 drought. The same end is gained in other delicate 

 herbs by the possession of an enlarged woody basal 

 portion. Then again, the tendency to general lig- 

 nification through all the parts of the plants 

 affords a capacity for resistance to many mem- 

 bers of the families Crueiferae and Compositae, 

 families known to us at home by their herbaceous, 

 unprotected representatives. To restrict evapora- 

 tion due to wind and solar radiation the desert 

 flora exhibits a high degree of reduction in the 

 surface area of its members. This principle is 

 illustrated in numerous instances by poverty of 

 foliage and considerable spininess, whilst in appar- 

 ent contradiction of this tendency one often finds 

 the surface of the plant clad in a hairy covering 

 or with glands and superficial excretions of wax 

 or resin or strongly aromatic substances. . . . 

 Further we find plants with smooth or shiny, thick 

 and fleshy, leaves. Nature does not work on one 

 plane, but provides for every case special means 

 of protection and fresh weapons to carry on the 

 struggle. Side by side with the thorn-bristling 

 Zilla spinosa we find the thick-leaved, wax-coated 

 Capparis spinosa, whilst near by are the hedgehog- 

 like Astragalus and Fagonia, and the soft, fleshy, 

 fiberless Mesenibrianihemum. In marked contrast, 

 too, are the Clienopodiaceae, a similar almost leaf- 

 less everlasting-woody throughout, and one would 

 think indestructible — and the delicate Parietaria 

 with its thin and battist-like foliage. Among the 

 life-destroying agencies of the desert, the omni- 

 present salt should be mentioned. 



Perennial plants are just about half as numerous 

 as the delicate annuals. Their existence is inde- 

 pendent of the fluctuating and variable annual 

 winter rains. They shoot anew and blossom even 

 after a rainless or all but rainless winter. In 

 marked contrast are the annual herbs which de- 

 pend absolutely upon the rainfall; nor is all rain 

 of equal value in promoting their development. 

 For a rich spring vegetation of annuals, the rain 

 should fall about the end of February and the 

 early part of March, at which time the growing 

 heat of the sun is capable of promoting germina- 

 tion. Trees are hardly met with in the district. 



As to the plants themselves, this flora pre- 

 sents some odd features. Thus we find only 



one true fern {Adiantum, capillus-veneris), and 

 the only gymnosperms are two species of 

 Ephedra. The grasses (Gramineae), legumes 

 (Leguminosae) and composites {Compositae) 

 are the larger families, there being 152 species 

 of the first, and 175 of the second, and 188 of 

 the third. The larger genera in these fam- 

 ilies are Panicum (14 sp.), Aristida (14), 

 Eragrostis (9), Bromus (10), Trigonella (12), 

 Medicago (16), Trifolium (15), Lotus (14), 

 Astragalus (28), Anthemis (10), Oentaurea 

 (16). Of Garex there are only 3 species, and 

 there are no orchids. Of Bosaceae there are 

 5 species, in as many genera. There is but 

 one species of Ericaceae. There is no species 

 of Solidago, nor even of Taraxacum. 



The tree-producing genera with which we 

 are familiar are mostly wanting, as Quercus, 

 Fagus, Acer, Ulmus, Fraxinus, which are not 

 represented, while Salix has 3 species, and 

 Populus 1. Tet Egypt is not lacking in tree 

 species, as witness the following list of genera, 

 each represented by one species unless other- 

 wise indicated: Phoenix, Cocos, Hyphaene, 

 Morus, Ficus (3), Acacia (6), Melia, Pistacia, 

 Mangifera, Zizyphus (2), Bhamnus, Sterculia, 

 Tamarix (6), Oarica, Elaeagnus, Eucalyptus, 

 Olea, Plumiera, Nerium. Many readers will 

 be surprised to learn that Bicinus communis 

 (the castor bean) is " an evergreen, usually 

 large shrub." 



The foregoing will give some idea as to the 

 interesting matter to be found in this impor- 

 tant addition to systematic and ecologie 

 botany. Charles E. Bessey 



The University or Nebraska 



Gas-Engine Principles. With Explanations 

 of the Operation, Parts, Installation Hand- 

 ling, Care and Maintenance of the Small 

 Stationary and Marine Engine, and Chap- 

 ters on the Effect, Location, Eemedy and 

 Prevention of Engine Troubles. By Rodger 

 B. Whitman. Published by D. Appleton 

 and Company, New York and London. 

 1912. 



As stated on the paper cover, " ' Gas-En- 

 gine Principles ' is a guide for the user of the 

 small stationary internal-combustion engine. 



