CHAPTER XX 



CLASSIFICATION OF ANGIOSPERMS AND SOME 



OF THEIR FAMILIES OF MOST ECONOMIC 



IMPORTANCE 



Classification. — The Angiosperms are so numerous and vary 

 so widely that their classification is not at all settled. Ray, a 

 noted English botanist (1628-1705), divided the Angiosperms 

 into two sub-classes — Monocotyledons and Dicotyledons — 

 on the basis of the number of cotyledons. There are also other 

 features which are used in distinguishing these two groups, 

 such as the number of floral structures composing the flower, 

 the venation of the leaves, the arrangement of the vascular 

 bundles in the stem, and the presence or absence of cambium. 

 Thus leaves with parallel veins, the parts of the flower in threes 

 or sixes, the scattered arrangement of vascular bundles m the 

 stem, and closed vascular bundles are characteristic of Mono- 

 cotyledons, while leaves with net-veins, floral parts in fours or 

 fives, vascular bundles arranged in a circle so as to enclose the 

 pith, and indefinite growth by means of a cambium are charac- 

 teristic of Dicotyledons. As to whether the Monocotyledons 

 arose from the Dicotyledons, or the Dictyledons from the 

 Monocotyledons is a question that botanists are not able to 

 answer satisfactorily. However, recent studies of the young 

 embryos of some of the Monocotyledons show that there are 

 two cotyledons present, one of which is very rudimentary. 

 This discovery with other structural and historical features has 

 given rise to the view that the monocotyledonous condition 

 arose from the dicotyledonous condition through the suppres- 

 sion of one of the cotyledons. This means that the gro\vth 

 which first becomes evident at the top of the developing embryo 

 as two points, each of which develops into a cotyledon in Dicoty- 

 ledons, became concentrated into the development of only one 

 point which consequently develops the single large cotyledon 

 characteristic of Monocotyledons. 



471 



