ANGIOSPERMS 369 



Classification of Angiosperms 



The Angiosperms agree so closely in their fundamental structure 

 as to leave little question that they form an entirely natural class. 

 With very few exceptions they readily fall into two series, Mono- 

 cotyledones and Dicotyledones. In the former, the embryo has the 

 first leaves alternate; i.e. a single cotyledon is developed. In the 

 Dicotyledons, the cotyledons are opposite. 



SUBCLASS I. MONOCOTYLEDONES 



The Monocotyledons are much less numerous, and, on the whole, 

 less specialized, than the Dicotyledons. There is greater uniformity 

 in the "tissues, and the structure of the flowers also shows less 

 variation. 



The simplest sporophyte is found in the Lemnacese, minute, float- 

 ing aquatics in which the sporophyte is, in Wolffia, a globular or 

 oval mass of tissue, without any external differentiation, but flat- 

 tened, and producing roots in Lemna. It is not entirely clear 

 whether the plant body in the Lemnacese is mainly a leaflike stem, 

 or a foliar structure. The largest Monocotyledons are the Palms, 

 some of which have an erect trunk fifty metres in height, with the 

 largest leaves found in any plants. The Rattan Palms (Calamus) 

 have slender, climbing stems of even greater length. 



The Monocotyledons are universally distributed, some forms, like 

 the Grasses, being cosmopolitan. A majority of the aquatic Sper- 

 matophytes are Monocotyledons, which play an important role in the 

 vegetation of marshes. The Reeds, Sedges, Bulrushes, etc., are all 

 Monocotyledons, and the same is true of the Pondweeds and most 

 other types of floating and submersed aquatics. In the sea there are 

 a number of characteristic types, especially in the Tropics. Of 

 these marine forms, Zostera and Phyllospadix may be mentioned 

 as American genera. Some of the fresh-water aquatic species occur 

 in great numbers, like Elodea Canadensis and Eichhornia crassipes 

 (" Water-hyacinth "). The latter, a floating plant, has become very 

 troublesome in some of our southern streams, where it was introduced 

 from the Tropics because of the beauty of its flowers. Of the terres- 

 trial Monocotyledons, the Grasses are the most widespread and abun- 

 dant. These are almost the only terrestrial monocotyledonous plants 

 which are sufficiently abundant, at least in temperate climates, to 

 give a decided character to the vegetation of any region. In the 

 warmer parts of the world, the Palms, and some of the treelike 

 Yuccas, and related forms are abundant enough to be very conspicu- 

 ous. This is especially true of the latter in dry regions like the 

 deserts of Arizona and Southern California, where the Yuccas and 

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