THE EVOLUTION OF PLANTS 225 



that the majority of the so-called " monocotyledonous " 

 grasses possess two cotyledons, one of which is more or less 

 rudimentary, and that the grasses are primitive monocoty- 

 ledons, representing a transitional stage from dicotyledons 

 to the higher monocotyledons. Monocotyledony, then, as 

 stated by Coulter, is simply one expression of a process 

 common to all cotyledony, gradually derived from dicotyle- 

 dony by reduction, and involving no abrupt transfer of a 

 lateral organ to a terminal origin. Variations in the rela- 

 tive size of the second cotyledon in grass embyros are 

 illustrated in Fig io6. 



Henslow^ was among the first to suggest the origin of 

 monocotyledons from dicotyledons. ^ Previous to the pub- 

 Hcation of his paper, it was generally assumed that mono- 

 cotyledons were the older group, and Henslow stated that 

 no systematist of his day recognized any real points of con- 

 nection between the two groups. He proposed the hy- 

 pothesis that the monocotyledons were derived by the 

 arrest of the development of one seed-leaf in a primitive 

 dicotyledonous Angiosperm;' hence said Henslow, "only 

 one elongates, its superior vigour carrying it on in a straight 



1 Henslow, Rev. George. A theoretical origin of endogens from e.xogens, 

 through self-adaptation to an aquatic habit. Journ. Linnean Soc. Boi. 

 19 : 485-528. May 15, 1893. 



' The first to make the suggestion appears to have been Agardh, in his 

 Larobok i Botanik, Part I. Malino, 1829-32. 



'In discussing the origin of Angiosperms, Arber {Journ. Linnean Soc. 

 Bot. 38 : 29-80. July, 1907) calls attention to the "Law of corresponding 

 stages in evolution," namely, that in the evolution of seed-plants, the stages 

 reached by different organs at any one period are dissimilar. From this 

 law it follows that such a plant as a "primitive Angiosperm," in the strict 

 sense of the term, that is, with all its organs primitive, never existed in 

 reality. We must picture the ancestors of modern Angiosperms as having 

 certain organs in a primitive stage of evolutionary development, others 

 as more advanced toward the stage in which they are now found. 

 IS. 



