6 BOTANIC GARDENS. 



ber of species. If these conditions alone determined the flora of 

 a region, the number of species which could be grown in a garden 

 would be determined only by its size and the number of plots it 

 might contain. It is found, however, that the substratum and 

 climate offer rigid limitations to an extension of the flora which 

 may be grown out of doors in any locality. The gardener par- 

 tially overcomes this limitation by the use of glass houses, where 

 plants from nearly all parts of the world may be grown in spe- 

 cially prepared soils, and kept at temperatures resembling those 

 of the natural habitats of the plants. But under such conditions 



The Main Palm House of the Royal Gardens at Kew, with Lake in Foregeound. 



it becomes extremely difficult to properly adjust the moisture and 

 light, and only a cornparatively small addition may thus be made 

 to the fiora of a garden. The conditions described above are such 

 that it has not been found possible to grow in one place more 

 than fifteen thousand species of the higher plants. It will be 

 found, moreover, that a large number of the species included are 

 not able to attain normal stature and appearance, and will thus be 

 useless in representing the form intended. 



In consequence of this limitation of the number it is custom- 

 ary to supplement the living plants by collections of prepared 

 specimens of contemporaneous and fossil forms, in order to rep- 

 resent more completely the vegetation of the globe. The living 

 as well as the prepared plants are generally so assembled as to 

 demonstrate the descent and relationship of the different groups, 

 distribution over climatic and geographic zones, as well as their 

 principal biological adaptations to the factors to be met in their 

 native habitats. In addition to this strictly natural method of 



