THE PAST HISTORY OF MONOCOTYLEDONS 233 



monocotyledons or vice versa, or from a stock capable of transmitting 

 either. 



Primitively we may consider that there was an adaptation from 

 aquatic to terrestrial conditions, which then gave rise to the 

 geophilous habit, whilst other forms in the harder struggle on land 

 retained or sought refuge again in an aquatic habit. 



The derivation of Angiosperms from the same synthetic type as 

 Gymnosperms seems clear. The near approach of the unique Bennet- 

 titeoe with two cotyledons and copious endosperm would alone suggest 

 this. The derivation of the latter from Pteridosperms is again more 

 or less clear. The fact that two cotyledons is so characteristic a 

 feature of Gymnosperms is again confirmatory of the early differentia- 

 tion of a dicotylous type. The polycotyledonary ^ forms are also 

 the more ancient, so that there is some evidence from analogy for 

 reduction. In Gymnosperms a suspensor is not always present. 

 Many of the Gymnosperms exhibit other characters, though these 

 may be homoplastic— ^.^^. parallel-venation and strap-shaped nature 

 of leaves, e.g. in Cordaites. 



It should be noted in regard to stem anatomy being a distinctive 

 •character, that the arborescent Lycopods sometimes, though not 

 invariably, exhibit cambial activity, whilst herbaceous forms do not. 



As to cotyledons the number is extremely variable amongst 

 Gymnosperms within the same group. So that although dicotyle- 

 dons and monocotyledons are remarkably uniform in this respect, yet 

 the character is apparently due to physical selection and is homo- 

 plastic, like the stem anatomy. 



Probably Dr. Worsdell's suggestion that the two are phases of one 

 group is nearer the mark. Amongst fossil Cephalopods the group 

 Nautiloidea is more or less constant in the central or dorsal position 

 in the siphuncle, whereas in Ammonoidea the siphuncle is ventral. 

 In Clymenioids, an Ammonite group, however, it is dorsal. 



Triassic Ammonoids, again, exhibit a curious type of sutural 

 development. In Nautiloidea Triassic Pleuronautilids exhibit what 

 may be called an homologous arrangement. 



Thus monocotyledons and dicotyledons may be compared with 

 Nautiloidea and Ammonoidea, and although the first two are plants, 

 and the last two animals, they exhibit a sort of parallel mode of 

 development, in respect of the degree and not the kind of difference. 

 The phenomenon is what Hyatt has described as morphic equivalents. 



Summing up the evidence in regard to the relative antiquity of 

 the monocotyledons and dicotyledons, and the bearing this has upon 

 the origin of the latter, we find ourselves reduced to the following 

 conclusions : — 



I. There is evidence from palaeontology that monocotyledons 

 came into existence almost contemporaneously with dicotyledons, 

 not, as once held, long prior to the latter. 



' There may be 3 to 18 cotyledons as in Finns, 3 lo S in Abies, 3 lo 11 in 

 Picea, 6 to II in Cednis. 



