PALEOBOTANY BERRY. 371 



phylogeny here advocated. This does not mean, however, that the 

 monocotyledons are strictly modern and monophyletic. The flowers 

 tend to be trimerous and chalazogamy has not been observed. There 

 is a greater reduction of sporogenous tissue in the megasporangium 

 than in the dicotyledons. There is one seed leaf or cotyledon which 

 is terminal in position, while the stem tip is lateral. The leaves are 

 poorly differentiated into blade and stalk and tend to be entire, sheath- 

 ing, and lack stipules. The venation is closed and the primary and 

 lesser systems are so sharply contrasted that the leaves are commonly 

 considered parallel veined, and even when the latter feature is less 

 obvious as in the arums and many tropical forms, the contrast in 

 caliber just mentioned remains obvious. 



In the dicotyledons the stem shows a tubular cylinder (siphonos- 

 tle) of collateral bundles, a persistent cambium forms secondary 

 phloem and xylem, both of which elements arise through cell fusion. 

 Secondary thickening results in a relatively larger display of assimi- 

 lative tissue (foliage) and a branching habit. The cotyledons are two 

 in number and lateral in position, and during germination the grow- 

 ing points and cotyledons are liberated by the elongation of the hypo- 

 cotyl as contrasted with the monocotyledonous tendency to free the 

 root and stem tip by the elongation of the cotyledon which functions 

 as an absorbing organ suggestive of the " foot " of pteridophytes. The 

 dicotyledonous flowers tend to be four or five merous. The leaves are 

 well differentiated into petiole, blade, and frequently stipules are 

 present. The venation is graduated and open and hence prevailingly 

 netted-veined, and this results in all degrees of lobation and division 

 of the laminae. 



Historically the oldest known angiosperms are found in the Lower 

 Cretaceous of Greenland, North America, Europe, Australia, and 

 New Zealand. These earliest known types do not appear to be 

 primitive (only the foliage and secondary wood structures are 

 known), and an extended earlier but unknown period of evolution 

 seems to be demanded. By the close of the Upper Cretaceous, both 

 monocotyledonous and dicotyledonous angiosperms are not uncom- 

 mon, and these include palms and most of the principal families ex- 

 cept the most specialized, such as the orchids among the former and 

 the composites among the latter. A second modernization of the 

 flowering plants took place at the dawn of the Tertiary and a third 

 modernization in the Miocene. The Miocene floras were much like 

 those of the present, but were richer in arborescent types and the 

 forms were more widely distributed than they are at the present 

 time. The bulk of the herbaceous vegetation appears to have been 

 relatively modern in its evolution and those large groups or parts 

 of groups that are especially characteristic of existing temperate 

 floras, such as the Cruciferae, Borraginaceae, Labiatae, Compositae, 



