THE JURASSIC FLOEA OF CAPE LISBUKNE, ALASKA. 46 



Oolite of Yorksliire, England, S to the I'ppcr Gondwanas of India, and 5 to the Jurassic of 

 California and Oregon. In the nearest continent, South America, there are no floras of any 

 importance that can be considered contemporaneous with this Antarctic flora. 



With the foregoing outhne of the distribution of Jurassic floras in ixiind, we are perliaps in a 

 position to draw certain tentative conclusions as to their probable avenues and means of dis- 

 pei"sal and climatic requirements. The systematic groups with which we mainly have to deal 

 are the FiUcales, Equisetales, Oycadales, Ginkgoales, and Coniferales. 



MEANS OF DISPEKSAL. 



The chief factors which are adduced to explain the distril)iiti(>n of living plants are animals, 

 especially birds and mammals: water, especially streams and ocean currents: air currents; and, 

 of course, human agencies. 



In considering the distribution of Jurassic plants we may at once eliminate mammals, 

 which were in Jurassic time on the threshold of their development, and birds, which were 

 represented by a single Icnown type (Archa?opteryx) that at best must have enjoyed but 

 limited powei-s of flight. There remain, therefore, only air and water transportation to account 

 for the distribution of Jui'assic plants. None of the plants is believed to have possessed any 

 marked mechanism for wide and systematic dispersal. 



The ferns of the Jurassic period are believed to have been homosporous, which implies 

 the production of vast numbers of spores of relatively short-lived vitality. A considerable 

 number of hving ferns having spores of this character are widely distributed on both hemi- 

 spheres. The male fern is an example; a single plant, according to Bower, may produce in 

 one season 5,000,000 spores, which are particularly well adapted to wind dispersal. That 

 ferns may pass over at least 25 miles of open water is shown by Treub, who found that on the 

 island of Karakatau, which had been completely wrecked and sterilized by a volcanic outburst, 

 within three years 12 species had already become reestablished. While the air currents may 

 have been competent to accomplish this, the possibihty of other agencies, such as transportation 

 by birds, ocean currents, or man, should not be ignored. 



The Equisetales were not an important element in the Jurassic flora, being mostly repre- 

 sented by fragmentary material referred to Equisetum or Equisetites. Although many of the 

 living species enjoy a wide distribution, little evidence is available as to their means of chspersal. 



The Cycadales are seed-bearing plants, the seeds being of small or mechum size. In the 

 living species the seeds sink in fresli water and presumably would do the same in salt water. 

 The length of time they would retain their vitality in salt water is problematical, though 

 probably it would be very short. Their transportation for any great distance by water is open 

 to (juestion. 



The Ginkgoales are very distinctly a waning type; in fact, they havi^ come almost to the 

 vanishing point, for the single li\ing species is not believed to exist in a purely wild state. 

 The identification of the early forms now commonly referred to the Ginkgoales is founded 

 mainly on the foliage organs, and their seeds are not certainly known, but since the discovery 

 of true Ginkgo in late Tiiassic or early Jurassic beds, and in practically its present form, the 

 seeds have not infi-equently been found. These seeds, like those of the living species, are of 

 large size. They have no special means for extended dispersal. 



In the living Coniferales there are certain minor devices for limited seed dispersal, such 

 as variously winged seeds, but nothmg that is especially remarkable. The seeds of Jurassic 

 Coniferales, so far as known, possessed no special devices for wider dissemination than that 

 usually taking place on a land surface. 



AVENtTES OF DISPEKSAL. 



The data presenlccl in the foi'egoing paragraphs regarding [ho means for disp(>rsal woidd 

 seem to indicate a practically continuous land connection between the several localities during 

 Jurassic time. W'ith the possible exception of the ferns, whose spores might have been trans- 



