296 VIEWS, &C. PHYSIOGNOMY OF PLANTS. 



Whether the aerial ocean surrounding the earth has always 

 exerted the same mean pressure is a question wholly unde- 

 cided. We do not even know for certain whether the mean 

 barometric height has remained the same during a hundred 

 years at any one given spot. According to the observations 

 of Poleui and Toaldo, this pressure appeared variable. Doubts 

 were long entertained regarding the accuracy of these views, 

 but the more recent investigations of the astronomer Carlini 

 render it almost probable that in Milan the mean barometric 

 pressure is on the decrease. Perhaps the phenomenon is very 

 local, and dependent on periodic variations in descending 

 currents of air. 



(15) p. 223 "Palms." 



It is remarkable, that of this majestic form of plants- 

 the Palms some of which rise to more than twice the height of 

 the Royal Palace at Berlin, and which the Indian, Amarasinha, 

 has very characteristically called " kings among grasses," 

 only fifteen species had been described up to the time of the 

 death of Linnaeus. The Peruvian travellers, Ruiz and Pavon, 

 added only eight; whilst Bonpland and myself, traversing 

 a greater extent of country, from 12 south lat. to 21 north 

 lat., described twenty new species, and distinguished as many 

 more which we named, without however being able to procure 

 their blossoms in a perfect state.* At present (forty-four 

 years after my return from Mexico) more than 440 species of 

 palms, from both continents, have already been scientifically 

 described, including the East Indian species arranged by 

 Griffith. The " Enumeratio Plantarum " of my friend K until, 

 which appeared in 1841, contains no fewer than 356 species. 



The very few palms belonging, like our Coniferae, Quer- 

 cine93, and Betulineae, to social plants, are the Mauritian 

 Palm (Mauritia flexuosa], and the two species of Chamaerops, 

 of which the Chamaerops humilis covers whole tracts of land 

 at the estuary of the Ebro and in Valencia, while the other, 

 Chamcerops Mocini, which we discovered on the Mexican 

 shore of the Pacific, is entirely without prickles. In the same 

 manner as there are some species of palms, including Cocos 

 and Chamaerops, which are peculiar to sea-coasts, so also is 

 there a certain group of Alpine palms belonging to the region 



* Humboldt, De distributione geographica Plantarum, pp. 225-233. 



