THE PLANT WORLD 81 



impression among those who may be well acquainted with some phases 

 of botany, but who have never given this question special consideration, 

 that the dicotyledonous plants have originated from monocotyledonous 

 ancestors, and this impression may be due in large measure to the greater 

 degree of differentiation attained by the Dicotyledons as compared with 

 the Monocotyledons as a group, and the assumption that the evolution- 

 ary chain is a simple and continuous one with the simpler members of 

 our flora representing the type of plants, although perhaps not the exact 

 species or genera, from which its more highly organized representatives 

 have evolved. But the problem is by no means so simple, for all the 

 groups of plants are not to be regarded as parts of a continuous evolu- 

 tionary chain, and there are many problems of exceeding complexity to be 

 solved before the relationship of all the groups — whether they be great 

 or small — are understood ; and the investigation which clears up these 

 dark points can not be superficial in character, for these cases of extreme 

 specialization and modification are neither few nor simple. 



We must not lose sight of these principles in considering the rela- 

 tionship of the two great groups into which the Angiosperms have been 

 divided. While the Monocotyledons are clearly less highly differentiated 

 than the Dicotyledons, the geological evidence does not seem to indicate 

 their greater antiquity. The two groups seem to have arisen almost 

 simultaneously or it has been suggested that the Dicotyledons are the 

 older of the two. In either case the geological evidence for the origin 

 of one group from the other is not great and, in case embryological evi- 

 dence can be found in favor of it, there is no objection to the formating 

 of the hypothesis that the Monocotyledons have originated from the 

 Dicotyledons, and in case it be shown that the Dicotyledons are really 

 the older it is certainly a point of weight in favor of the new theory. 



It would be a serious omission to fail to mention a theory which Mr. 

 Lyon, whose study of the development of the Nelumbium embryo has 

 been a stimulus to a more careful consideration of the relationship of 

 these groups, has recently advanced. Cotyledons are generally regarded 

 as modified leaves. No suggestion to the contrary is to be found in our 

 general text-books of botany and only very rarely in the more exhaust- 

 ive and technical works. In their behavior during germination many of 

 them resemble in the closest way foliage leaves, though usually quite 

 different from the foliage leaves of the plant upon which they grow ; but 

 many are so much modified as to have lost all resemblance to a leaf, 

 as is well illustrated in the absorbing organ of the grasses, palms, and 

 others. The theory proposed by Mr. Lyon is that the cotyledon is not 

 homologous with the leaf, but with the nursing foot of the mosses 

 and ferns. If this theory be the correct one we must regard the coty- 

 ledons of the grasses, palms and other Monocotyledons as the primitive 



