636 BOTANY 



other groups, and it is not until the upper Jurassic formations, and 

 possibly still later, that fossils are encountered which can without 

 question be assigned to the Angiosperms. 



To judge from a study of the living forms, it is likely that the 

 earliest members of the class, especially among the Monocotyledons, 

 were plants of delicate structure, which were not fitted to leave fos- 

 sil traces ; and this makes it probable that the first Angiosperms 

 appeared in somewhat earlier formations than those in which their 

 first fossil traces are found. 



Monocotyledons. The Monocotyledons are largely plants without 

 woody stems, and with delicate leaves, of such simple structure as 

 to make their identification in a fossil state extremely difficult. A 

 few types, like the Palms, have firm leaves, which are often met with 

 fossil, but these are the exception. 



Probably the oldest known fossil which is an unmistakable Mono- 

 cotyledon is Alismacites, of which the leaf-prints are much like 

 Alisma and other related forms. These prints occur in the lower 

 Cretaceous of Portugal. Palms are first encountered in the Ceno- 

 manian, middle Cretaceous, and in the upper Cretaceous they were 

 evidently abundant, though less so than in the Tertiary, where 

 examples of most of the existing types of Monocotyledons occur. 



Dicotyledons. While the lower Monocotyledons are mostly her- 

 baceous plants, this is not true of the primitive Dicotyledons, many 

 of which are shrubs and trees, with firm leaves well fitted to leave 

 fossil remains. With few exceptions, the earliest fossil Dicotyle- 

 dons may be assigned to living families, and often to existing genera. 

 The age of these earliest Dicotyledons is about the same as that of 

 the Monocotyledons, as they are found in the lower Cretaceous. The 

 oldest known forms belong to the Apetalse, species of Willow and 

 Poplar occurring in the lower Cretaceous, and Oaks, Beeches, Chest- 

 nuts, and Birches being found in the upper Cretaceous. 



During the Tertiary, as in the case of the Monocotyledons, the 

 increase in the number and variety of the Dicotyledons was very great, 

 and most of the families were probably represented. It is an interest- 

 ing fact that the great family of the Compositae, generally recognized 

 as the most specialized of the Dicotyledons, and presumably one of 

 the most recent, is very poorly represented in a fossil condition, and 

 these are all from the later Tertiary and more recent formations. 



FACTORS INFLUENCING THE DISTRIBUTION OF LIVING 



PLANTS 



In considering the present distribution of plants upon the earth, 

 several factors must be taken into account. First of all, we must 

 consider the distribution of the ancestors of the modern flora as 



