CHAPTER XXX 



SEED-BEARING PLANTS (Concluded) 



MONOCOTYLEDONS 



425. General Characters. — The monocotyledons are, 

 in almost every respect, of simpler structure than the 

 dicotyledons. As the name indicates, the embryo has 

 only one cotyledon; the parts of the flower are usually 

 in threes or sixes, but never in fives, as in dicotyledons; 

 the leaves are, with rare exceptions, parallel-veined, 

 and the early ones are always alternate on the stem. A 

 cross-section of the stem shows that the fibro-vascular 

 bundles are not arranged in a circle about a central 

 pith (exogenous type), but are distributed irregularly 

 throughout the parenchyma (endogenous). There is no 

 layer of perennial cambium, and consequently no cylinders 

 of wood and bark are formed each growing season, as in 

 the dicotyledons. The general characters of the group 

 are illustrated diagrammatically in Fig. ^;^S. 



426. Relation to Dicotyledons. — A comparison of the 

 monocotyledons with the more highly developed dicoty- 

 ledons raises at once the question as to whether the former 

 are the more ancient forms from 'which the dicotyledons 

 have been evolved, or whether dicotyledons are the more 

 primitive, in order of development, and the monocotyle- 

 dons derived from them by reduction and simplification. 

 There is evidence on both sides of this question, which will 



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