332 SUMMARY OP THE CONTENTS OF VOL. II. 



from 1800 to 2000 years. The root of the rose tree growing 

 against the crypt of the Cathedral of Hildesheim is 800 years old. 

 A kind of sea-weed, Macrocystis pyrifera, attains a length of 630 

 English feet, exceeding therefore the height of the loftiest Coni- 

 ferae, even that of the Sequoia gigantea .... 94 97 



Examination of the probable number of phsenogamous plants hitherto 

 described or preserved in herbariums. Relative numbers. Laws 

 discovered in the geographical distribution of plants. Relative num- 

 bers of the great divisions of Cryptogamia to Cotyledonous plants, 

 and of Monocotyledonous to Dicotyledonous plants, in the torrid, 

 temperate, and frigid zones. Elements of arithmetical botany. 

 Number of individuals ; predominance of social plants. The forms 

 of organic beings are mutually dependent on and limit each other. 

 If we know exactly the number of species of one of the great 

 families of Glumacese, Leguminosse, or Compositse, at any one 

 part of the globe, we may infer approximatively both the number 

 of species in the remaining families, and the entire number of 

 phsenogamous plants in the same district. Application of the 

 numerical ratios to the direction of the isothermal lines. Mys- 

 terious original distribution of types. Absence of Roses in 

 the southern, and of Calceolarias in the northern hemisphere. 

 Why has our heather (Calluna vulgaris), and why have our oaks 

 never advanced eastward beyond the Ural Mountains into Asia ? 

 The vegetation cycle of each species requires for its successful 

 organic development a certain minimum amount of temperature. 



97113 



Analogy between the numerical laws of the distribution of animal and 

 of vegetable forms. If there are now cultivated in Europe above 

 35000 species of phsenogamous plants, and if our herbariums pro- 

 bably contain, described and undescribed, from 160000 to 212000 

 species of phsenogamous plants, it is probable that the number of 

 collected insects and collected phsenogamous plants are nearly 



