8o AN INTRODUCTION TO THE STUDY OF FOSSILS 



since the vascular bundles formed in spring have thinner walls 

 than those formed in late summer and fall the annual growth 

 becomes visible as a concentric ring. A taproot is usually 

 present ; the parts 'of flowers are in fours or fives. 



The dicotyledons are now generally regarded as more 

 primitive than the monocotyledons and as their probable 

 ancestor. 



(i) One of the most primitive of the dicotyledons is the 

 American tulip tree, Liriodendron tuHpifera, a beautiful forest 

 type, of eastern North America, reaching a diameter of four to 

 twelve feet, and a height of sixty to one hundred and ninety 

 feet. The closely related species, L. chinensis, occurs in eastern 

 Asia. These are the sole survivors of the genus Liriodendron 

 which had formerly a far wider distribution. Appearing first 

 in the Cretaceous in great variety and number of species, it 

 flourished during this period in North America from Wyoming 

 to New Jersey and in Greenland. In Tertiary time it still ex- 

 tended its range over Iceland and Eurasia ; but the cold climates 

 of the Pleistocene forced the last remnants of Liriodendron 

 southward in Europe and finally cut them off on the shores of 

 the Mediterranean Sea, which, with the Alps, acted as a trans- 

 verse barrier to the farther retreat of this and many other types. 

 Thus is explained the paucity of dicotyledons in Europe as 

 compared with North America and eastern Asia, where retreat 

 southward during the ice age was not thus cut off by high moun- 

 tains or transverse shores. This late restriction of distribution 

 is also seen in Engelhardtia (Fig. 29). 



(2) Sassafras, like Liriodendron, is also a ditypic genus now 

 restricted to S. officinalis, confined to eastern North America, 

 and a closely related Chinese form. Like Liriodendron, it is 

 the last representative of a large group which flourished at least 

 throughout North America and Europe since the Comanchean, 

 but could survive the cold Pleistocene climate only in North 

 America and eastern Asia. It is known first from the Coman- 

 chean of Virginia, Greenland and Portugal, later from the 



