246 Plant Life and Evolution 



Azaleas and Rhododendrons, avoid soils containing 

 lime, while for many other plants lime is an essen- 

 tial. The mechanical characters of the soil, that is, 

 whether it is compact or loose, retentive of water 

 or the reverse, are also important factors in deter- 

 mining the distribution of many plants. Finally 

 the special devices, like winged seeds and fruits, 

 play an important part in determining the distribu- 

 tion of some plants. Most weeds are such because 

 of the facility with which they can be distributed. 

 With this facility for distribution there also goes 

 the hardiness and adaptability which these plants 

 exhibit. One has but to contrast the dandelion with 

 such an orchid as Arethusa, for example, to realize 

 the difference between a really adaptable plant and 

 an exceptionally particular one. 



Ancient Distribution of Land. There are still 

 evident some traces of the ancient divisions of the 

 land areas of the world into a northern and south- 

 ern mass, shown by the character of the vegetation. 

 A good many families of plants are still confined 

 respectively to the Northern and Southern Hemi- 

 sphere. Thus many conifers, the pines, firs, etc., are 

 distinctly northern types. The Araucarias and 

 Kauri pine (Agathis) are equally characteristic 

 southern coniferous types. Among the angiosperms, 

 willows, oaks, birches, and maples are examples of 

 families practically confined to the Northern Hemi- 

 sphere. The very peculiar Casuarina, sometimes 

 cultivated in California, and the Proteaceae, of 



