TFIE PAST HISTORY OF MONOCOTYLEDONS 22 1 



as the older group. Professor Jeffrey finds cambial activity in 

 monocotyledonous seedlings in their third period when they become 

 ascending axes, and considers they are derived from dicotyledons 

 by adaptation to an amphibious habit. 



The single cotyledon is terminal, but, if derived from dicotyledons, 

 is thought to be only apparently terminal, really lateral. In 

 Delphinium 7iudicaule (see PI. III. figs. 2 and 3) at eight weeks the 

 cotyledon is terminal, but at fourteen weeks the plumule is. In 

 A/isma (PI. III. fig. 4, A), Tamus communis {ibid., 4, D), and the 

 pseudomonocotyledon Corydalis cava {ibid., 4, B, C) the cotyledon is 

 formed before the plumule and is terminal; but when the plumule is 

 developed the cotyledon is forced to one side. This is explained 

 as due to the early formation of the plumule, or to the partial union 

 of one margin only, as in Ranunculus ficaria. 



The other characters, parallel venation of leaf, short duration 

 of primary root, albuminous seeds, trimerous symmetry — except the 

 last — are regarded as adaptations to mode of life. 



Miss Sargant reviews the theory of Professor Henslow. This is 

 based largely on the great number of aquatic monocotyledonous orders. 

 He considers monocotyledons are derived from dicotyledons by 

 suppression of one cotyledon, due to their aquatic habit. But 

 aquatics and geophytes have characters in common. Vascular 

 structure is reduced by an aquatic habit. Parallel venation is 

 found only in aquatic monocotyledons. Tufted roots are found in 

 aquatic monocotyledons and dicotyledons. Aquatic dicotyledons 

 do not tend to become monocotylous nor produce albuminous 

 seeds. The fioral structure is not uniform, and may be derived from 

 an ancestor between Ranales and Liliiflorge. Probably aquatic 

 monocotyledons were not primitive but driven by severe competition 

 on land to the water, where conditions are more uniform. Aquatic 

 conditions tend to modify vegetative organs, but not the flower. 

 This is because it is a physiological phase. 



Geophytes can adapt themselves either to an aquatic or a 

 climbing habit. Arboreal monocotyledons are not aquatic, but 

 possess anomalous thickening rings as in Aloe, Draccena, etc. 

 Palms, again, recall the characters of geophilous plants, being 

 semi-bulbous. 



Of phylogenetic schemes, which embrace the other characters, 

 eg. parallel venation, trimerous floral structure, ephemeral primary 

 root, albuminous seeds, etc., there is Professor Lyon's theory of the 

 origin of dicotyledons from monocotyledons by fission of one to 

 form two cotyledons, of use to the cotyledon in forcing itself from 

 the seed. Agardh first proposed the alternative theory, but only 

 applied it to a few monocotyledons. 



Professor G. Henslow extended this theory, deriving monocoty- 

 ledons from dicotyledons by suppression of the one cotyledon due 

 to the aquatic habit. This is based to a great extent on the large 

 proportion (33 per cent.) of aquatic orders. Miss Sargant regards 

 monocotyledons as a decadent race driven to an aquatic habit to 



