THE MONOCOTYLEDONS 415 



that the monocotyledonous angiosperms have been derived from 

 a dicotyledonous ancestry. The justice of this hypothesis, how- 

 ever, will be finally established only when we shall have at our 

 disposal for anatomical investigation remains of monocotyledons 

 from Mesozoic deposits. Whether or not it is ultimately estab- 

 lished that the large group at present under discussion has actually 

 been derived from the dicotyledons, it will doubtless in any case 

 be clear that they cannot in any way be regarded as primitive 

 representatives of the angiosperms. The monocotyledons in 

 fact represent the herbaceous type in its extremest form. In 

 the group the fibrovascular tissues are released from the rigid 

 confinement of the tubular stele in the ancestral forms with sec- 

 ondary growth by the development of large parenchymatous 

 storage devices in relation to the foliar traces. Correlated with 

 this release is the possibility of accommodation of the numerous 

 foliar strands which characterize the basal regions of monocotyledo- 

 nous leaves throughout the transverse section of the stem. There 

 is doubtless some correlation also between the extreme multipli- 

 cation and consequent displacement of the strands in the stem 

 of monocotyledons and the disappearance of secondary growth. 

 Possibly an aquatic or amphibious habitat long maintained may 

 likewise have acted as a contributory cause in bringing about the 

 obliteration of cambial activity, since in the dicotyledonous 

 Nymphaceae, in which there is diffuse distribution of the bundles 

 and also the absence of cambial activity, we find these features 

 correlated with an aquatic habitat. It has, indeed, often been 

 suggested that the Nymphaceae or Ranunculaceae are the dicoty- 

 ledonous ancestors of the monocotyledons. The interesting inves- 

 tigations of Sargent on the fusion of the cotyledonary structures 

 in Ranunculus ficaria, etc., are of importance in indicating how 

 monocotyledony may have arisen as a result of the union of two 

 originally separate seed leaves. Another possibility, of course, is 

 the origin of the monocotyledonous embryo as a consequence of 

 the abortion of one of the two original cotyledons, and this view 

 is perhaps supported by the conditions found in certain grasses, 

 such as Zizania, Avena, etc., in which the vestige of a second 

 cotyledon is considered to be present. The problem of the origin 



