THE MESEOSERE. 



413 



cycads were of course abundant and diversified, whence it has been called the 

 age of cycads. The flora is remarkably uniform over wide portions of the 

 world. Thus not far from 50 per cent of the North American flora^xclusiye 

 of the cycad trunks — ^is the same as that found in Japan, Manchuria, Siberia, 

 Spitzbergen, Scandinavia, or England, and, what is even more remarkable, 

 the plants found in Louis Phillippe Land, 63° S., are practically the same as 

 those from Yorkshire, England. 



"Some idea of the cUmatic conditions which prevailed at this time may be 

 gained from the present distribution of certain obvious descendants of the 

 Jurassic flora. Thus Matonidium and Laccopteris are represented by Matonia, 

 of which there are two species Uving in the Malay region and in Borneo; 

 Dictyophyllum, Protorhipis, Hausmannia, Caulopteris, etc., are closely related 

 to Dipteris, which has five species living in the eastern tropics; Ginkgo — so 

 abundant in the Jurassic — has but a single living representative in China and 

 Japan. 



"The presence of luxuriant ferns, many of them tree-ferns, equisetums of 

 large size, conifers, the descendants of which are now found in southern lands, 

 all point to a moist, warm, probably subtropical climate, though in late Jurassic 

 time the presence of well-defined rings in the tree-trunks of species found in 

 northern areas — King Karl's Land, Spitzbergen, etc. — shows that there were 

 beginning to be sharply marked seasons." 



Some idea of the vegetation of the Pacific coast and its serai and climax 

 differentiation during the Jurassic may be gained from the following list 

 (Knowlton, 1910^:43): 



Bryophyta: Marchantites. 



Pteridophyta: Dicksonia, Coniopteris, Thyrsopteris, Polypodium, Cladophlebis, Sclerop- 



teris, Ruffordia, Adiantites, Taeniopteris, Macrotaeniopteris, Angiopteridium, 



Sagenopteris, Danaeopsis, Hausmannia, Onychiopsis, Equisetum (?). 

 Cycadales: Ptilozamites, Nilsonia, Pterophyllum, Ctenis, Ctenophyllum, Podozamites, 



Otozamites, Encephalartopsis, Cycadeospermum, Williamsonia. 

 Ginkgoales: Ginkgo, Baiera. 

 Coniferales: Taxites, Brachyphyllum, Pagiophyllum, Araucaritea, Pinus, Sequoia, Cyclo- 



pitys, Sphenolepidium. 



The Cycadean climax. — The r61e of cycads in existing vegetation is assimied 

 to indicate or reflect the r61e of the cycadeans in the Jurassic period. To-day 

 they occur as xerophytic scrub {Stangeria, Microcycus, Dioon), as constituents 

 of mesophytic or hydrophytic subtropical or tropical forest (Dioon, Cycas), 

 and as a grass-like undergrowth in subtropical coniferous forests (Zamia). 

 They are thought to have occurred in similar relations during the Jurassic. 

 Hence, the assmnption is made that a cycadean climax of xerophytic scrub 

 existed over the central portion of the Great Plains, and perhaps in California 

 and Oregon also. This climax is supposed to have been inclosed by an 

 araucarian climax, and it may have been bordered by a boreal coniferous 

 forest in the north. It is also thought that cycadeans played a part in the 

 development of the coniferous cUmaxes, and hence persisted as undergrowth 

 or reUct communities. From the extremely suggestive account of Wieland 

 (1906), it would be possible to reconstruct the developmental relations of the 

 cycadean cUmax in some detail. Such an attempt would be premature 

 beyond pointing out that equiseta and ferns were still the outstanding domi- 

 nants of medial associes, and that the cycadeans must have served sometimes 

 as serai dominants in the coniferous climaxes, as well as climax dominants 



