THE HISTORY OF PLANTS 



415 



pendently from some of the Paleozoic seed ferns, and both have swimming 

 sperms within the pollen tube, reminiscent of the free-swimming sperms 

 of their still more remote fern ancestors. Not all the early Mesozoic forests 

 were scrublike ; in some places there were dense stands of tall pines. Some 

 of the silicified pine logs of the Jurassic Petrified Forest in Arizona are 

 10 feet in diameter and 100 feet long and give evidence of trees that 

 towered to a height of nearly 200 feet. A striking feature of the Jurassic 

 flora is the wide distribution of many of its species; excluding the cycads, 

 the same kinds of plants occurred in the New and Old Worlds and from 

 Alaska and Spitsbergen to the antarc- 

 tic coasts. Obviously Jurassic climates 

 were mild even in high latitudes. 



The rise of flowering plants. One 

 of the most dramatic events in the 

 history of life was the sudden rise of the 

 angiosperms, or flowering plants, dur- 

 ing the Cretaceous period. No one has 

 succeeded in finding their ancestors, 

 and their origin is a mystery. Their 

 oldest known fossils, from the early 

 Cretaceous, are already trees of types 

 now familiar — magnolia, fig, sassafras, 

 and poplar, among the dicotyledons, 

 and palms, representing the monocot- 

 yledons. By mid-Cretaceous times the 

 forests had become essentially modern, 

 including such other trees as beech, 

 birch, willow, maple, oak, walnut, sycamore, tulip, sweet gum, bread- 

 fruit, and ebony, together with shrubs like laurel, viburnum, ivy, hazel, 

 and holly Pines and other conifers persisted as in modern times, but they 

 no longer dominated the landscapes as in the preceding period. The forest 

 undergrowth was still made up of ferns and lycopods; herbaceous angio- 

 sperms did not appear until later, and the dry plains were still probably 

 barren and desertlike in the absence of grasses. 



The coming of the angiosperms had a profound effect upon the evolu- 

 tion of terrestrial vertebrates. All animal life is ultimately dependent 

 upon plant food; but prior to the Cretaceous nearly all land vertebrates 

 were carnivores — members of food chains in which larger organisms 

 preyed upon smaller, and so on down to the smallest, which were the 

 insects and other tiny creatures that fed on plants and plant debris. 

 Under such circumstances the larger forms could never have been abun- 

 dant. It is true that some of the Jurassic reptiles did become adapted to 

 feeding upon the coarse leaves of cycads and other gymnosperms and 



Fig. 26.15. A relic from the Mesozoic. 

 The maidenhair tree, Gingko biloba, lone 

 survivor of its group. (Redrawn from 

 Strassburger, Textbook of Botany, by per- 

 mission The Macmillan Company.) 



