CHAPTER IX 

 SPERMATOPHYTA (CONTINUED) 



2. ANGIOSPERMAE 



The angiosperms are the largest and most conspicuous group of modern 

 plants, numbering about 195,000 species. They are also the youngest 

 group, and are thought to have been derived from the gymnosperms, or 

 possibly to have had an independent origin from the pteridophytes. 

 They appeared in the Lower Cretaceous and from the Upper Cretaceous 

 to the present time have been the dominant group of land plants (Fig. 

 258). Angiosperms are found in practically all terrestrial habitats where 

 plants may exist. Some occur in fresh water and even in the sea, the 

 aquatic habit being secondarily acquired. A relatively few forms, having 

 little or no chlorophyll, are saprophytic or parasitic. 



Although many angiosperms are woody, the majority are herbaceous. 

 The woody condition is considered to be the more primitive, and the 

 herbaceous one to have been derived from it. The seeds of angio- 

 sperms are borne in a closed vessel, the ovary, and not, as in gymno- 

 sperms, on the exposed face of an open carpel (or equivalent structure). 

 The ovary represents the basal portion of a single closed carpel or of two 

 or more united carpels. It ripens to form a fruit, which contains the 

 seeds. Angiosperms are often called "flowering plants," as the presence 

 of flowers is one of their most outstanding features. 



The two great groups (subclasses) of angiosperms are the Dicotyle- 

 doneae and the Monocotyledoneae, distinguishable on the basis of the 

 following combination of characters, but with individual exceptions to 

 each: The dicotyledons have seeds with two cotyledons, stems with a 

 hollow cylinder of vascular tissue and with a functioning cambium, leaves 

 with netted veins forming an open system, and floral parts chiefly in fours 

 or fives. The monocotyledons have seeds with one cotyledon, stems with 

 scattered vascular bundles and without a functioning cambium, leaves 

 with parallel veins forming a closed system, and floral parts typically in 

 threes. 



The Dicotyledoneae include 240 families and approximately 155,000 

 species, the Monocotyledoneae 45 families and about 40,000 species. 

 The Dicotyledoneae comprise the Archichlamydeae, whose flowers are 

 naked, apetalous, or choripetalous, and the Metachlamydeae, whose 

 flowers are sympetalous. The Archichlamydeae include some members 



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