in the Distribution of Vegetable Forms. 283 
crease towaMs the frigid zone. The geographical distribution 
of ferns depends ii^h the union of the local circuihstahces of 
shade, humidity, and moderate heat. Their maximum (that is 
to say, (he place where the denominator of the rioraial fraction 
of a group becomes the least possible,) is fouiid in the moun- 
tainous parts of iiie tropics, particularly in islands of small ex- 
tent, where the p'ropbrtion rises to J, ahd upwards. When the 
plains and mountains were" not separated, Mr BVown found the 
ferns of the torrid zone to be 5 ^ 5 . Iii Arabia, India, New Hol- 
land, and Western Africa, (between the tropics,) they are 5 ’^ ; 
our herbaries of America do not ^ve more than ; but the 
ferns are rare in the great valleys and on the dry platforms of 
the Andes, Where we were forced to remain a long time. 
{Congo, p. 43., and Novagen., vol. i. p. 33^) In the tempe- 
rate zone, the ferns are y ^ ; in France, ; in Germany, ac- 
cording to late inquiries, {Bert, Jahrb,, b. i. p, Sd.) The 
group of ferns is extremely rare on the Atlas mountaihs, and 
almost completely disappears in Egypt. In the frigid zone, 
the ferns appear to rise to 
Monocotyledones. The denominator becomes progres- 
sively smaller in proceeding from the equator toward the 62 d 
degree of North Latitude ; it increases again in the regions still 
farther north, on the coast of Greenland, where the Gramineae 
are very scanty. {Congo, p. 10.) In the different parts of the 
tropics, the proportion varies from J to J. In 3880 phaeno- 
gamous plants of equinoctial America, found by M. Bonpland 
and me, in flower and in fruit, there are 654 monoctyledonous, 
and 3226 dicotyledonous ; hence the great division of the Mo- 
nocotyledones would be J of the phaeiioganious plants. Ac- 
cording to Mr Brown, the proportion in the Old Continent (in 
India, equinoctial Africa, and New Holland,) is J ; in the 
temperate zone we find J. (France, 1 : 4| ; Germany, 1 : 4 | ; 
North America, according to Pursh, 1 : 4| ; Kingdom of Na- 
ples, 1 : 4i; Switzerland, 1 : 4 J ; British Isles, 1 : 3f ) ; in 
the frigid zone, 
Glumace^. (The three families of Juncci3e, Cypcraceee, 
and Gramineae, united). =:Trop., Temp., Frig,, 
