464 SUMMARY OF THE CONTENTS. 



Colossal size and great age of some kinds of trees ; Dragon tree of 

 Orotava thirteen, and Adansonia digitata (Baobab) thirty-two Eng- 

 lish feet in diameter. Characters cut in the bark of the trees in 

 the 15th century. Adanson assigns to some of the Baobab trunks 

 in Senegarnbia an age of between 5100 and 6000 years 283288 



Judging by the annular rings, there are yew-trees (Taxus baccata) 

 from 2600 to 3000 years old. Is it true that, in the northern tem- 

 perate zone, the part of the tree turned towards the north has nar- 

 rower annular rings, as Michel Montaigne affirmed in 1581 ? Species 

 of trees in which individuals attain a size of above twenty-one or 

 twenty-two English feet diameter, and an age of several centuries, 

 belong to the most different natural families . . 288 289 



Diameter of the Mexican Schubertia disticha of Santa Maria del Tule 

 40 J English feet ; the sacred Banyan fig-tree of Ceylon almost 30 ; 

 and the oak at Saintes (Dep. de la Charente Inf6rieure) 29 J English 

 feet. The age of the oak tree estimated from its annular rings at 

 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 Coniferae, even 

 that of the Sequoia gigantea . . . . . 289291 



Examination of the probable number of phaenogamous plants hitherto 

 described or preserved in herbariums. Relative numbers. Laws 

 discovered in the geographical distribution of plants. Relative 



) numbers 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 bota- 

 ny. 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, Leguminosae, or Compositae, 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 nu- 

 merical ratios to the direction of the isothermal lines. Mysterious 

 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 develop- 

 ment a certain minimum amount of temperature . . 291 303 



Analogy between the numerical laws of the distribution of animal and 

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

 35,000 species of phsenogamous plants, and if our herbariums pro- 

 bably contain, described and undescribed, from 160,000 to 212,000 

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

 collected insects and collected phaenogamous plants are nearly 



