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JOUENAL OF THE ROYAL HORTICULTURAL SOCIETY. 



blade of Ginkgo and in the Sphenophylluros. Even in those of 

 Cordaites, the ribs of which seem to be parallel, the veins are described 

 both by Dr. Scott* and Miss Stopesf as dichotomoiis. 



The ribs themselves probably branch from the base, though running 

 subsequently parallel, just as in the phyllodes of Lathyrus Nissolia, 

 Oxalis bupleurifolia and species of Australian Acacia. 



These long " leaves " with parallel ribs of the Cordaites have been 

 compared with those of Monocotyledons, but the intermediate vena- 

 tion is different. In the latter class the ribs are mostly joined by hori- 

 zontal cross-bars at right angles, apparently to secure strength, for the 

 blade is weakened by the influence of water; whereas in phyllodes of 

 Dicotyledons the veins start at an acute angle, and when they are broad 

 the veins become reticulated. 



Hence, it seems probable that as the Gordmtes are xerophytes, the 

 supposed leaves may be really phyllodes, like those of Acacia. The 

 vertical, instead of horizontal, position of the latter is probably due to 

 the necessity of avoiding injury from the loss of heat by radiation. :|: 



We will now leave the primary and secondary epochs and come dow^n 

 to the Miocene of the Tertiary epoch, of which fossil plants have been 

 found in various parts of the northern regions of the globe. 



At Oeningen, in North Switzerland, 465 species are known, of 

 which 166 are trees and shrubs, including many living American 

 genera, as Sequoia (the "Big-tree," or Wellingtonia of California), 

 Oaks, Liriodendron (the Tulip tree). Maple, Plane, &c. At Mull and 

 Bovey Tracey in Devonshire, Sequoia also occurs. 



On the West Coast of Greenland (70° N. Lat.) a fossil flora exists, 

 including the Walnut, Vine, Magnolia, &c. Even at Grinnell Land 

 (80'^ N. Lat.) off the West Coast of Greenland, are the Norway Spruce, 

 the deciduous cypress {Taxodium) and Sequoia. Another site is 

 Colorado. Now, omitting the more Arctic districts, all these sites of 

 the fossil Miocene, viz., Colorado, British Isles, and Oeningen, lie 

 between 30^ and 60° N. Lat., while the existing floras characterized by 

 having descendants of the Miocene, viz., East Coast of North America 

 and Japan, lie between 20° and 40° N. Lat. 



The question arises, how did they get to these two far distant 

 countries ? There are about 100 genera common to both ; seventy-seven 

 genera are also common to the Swiss Miocene, of which twenty-six are 

 not now living in Europe. Iceland and Greenland have them as well. 



It appears, then, that in the early Miocene period, or perhaps in 

 the latter part of the preceding Eocene, or the intermediate " Oligo- 

 cene," times, the Arctic regions were favoured with a temperature now 

 prevailing in the warmer temperate zone, and as the cold drew on they 

 were driven southwards, where land w^as continuous, till they settled 

 along the 40th parallel of latitude. Obstructions, such as the mountains 

 * New Phytologist, vol. ii. p. 92, 



t "A Theoretical Origin of Endogens," Joi/rn. Lin. Soc. Bot. xxix. p. 485. 



X I have elsewhere suggested that the long parallel veined "leaves" of 

 Monocotyledon are all really phyllodes, the blades being restored in Saglttaria, 

 Ta7nvs, &o. Loc. clt. p. 517. 



