THE PAST HISTORY OF MONOCOTYLEDONS 225 



that dicotylar forms gave rise to monocotylous by fusion of the two 

 cotyledons. In Ranales, e.g. Erant/iis^ the root is diarch, and tetrarch 

 structure is found in the hypocotyl. In Fodophyliiwt peltahim the 

 same occurs. Aneniarrhena and Althea are recalled in these. In 

 both the cotyledons are united for some distance. Reduction of 

 the tetrarch type has progressed further than in Althea, of which 

 Afiemarrhena is a reduced form, and with it Podophyllum and 

 Era)ithis agree, showing that the union of cotyledons in a tetrarch 

 dicotyledon may result in a monocotylous type with structure of 

 Anemarrhena. 



Professor G. Henslow, in a paper ^ read before the Linnean Society, 

 brought forward the theory of the origin of endogens from exogens 

 by adaptation to an aquatic habitat due to suppression of one 

 cotyledon. This theory, which is an extension of Agardh's, is based 

 upon the large proportion of monocotylous aquatic orders in the 

 group, viz. 33 per cent. There are, however, some characters which 

 are found in both monocotyledons and dicotyledons, and those which 

 are thought to be confined to aquatic plants alone are also found in 

 geophytes and other groups. Aquatic conditions have certainly 

 helped to reduce the stem-structure and to modify foliage, but are 

 not responsible for the reduction in cotyledons, nor so far as is 

 known, the character of the flowers. 



Messrs. E. A. N. Arber and J. Parkin,"-^ in a paper on the " Origin^ 

 of Angiosperms," trace the ancestry of the group. They disagree 

 with Engler's view that the present-day primitive Angiosperms have 

 unisexual flowers, e.g. Piperales, Pandanales, etc. But the perianth 

 is to be regarded as arising de novo, and to be an organ sui generis. 



The plants cited have a sharply defined complicated structure. 

 This theory has proved barren. The strobiloid theory is held to 

 solve the question, the primitive form being amphisporangiate with 

 micro- and megasporophylls and a well-defined perianth. The parts 

 of the flower were numerous, arranged spirally, the female sporo- 

 phylls above the male. Such a type is found in some Magnoliaceae, 

 Ranunculaceae, Alismaceas. The apetalous unisexual flowers are 

 regarded as derived from these. The flower is an anthostrobilus of 

 which a gymnospermic and an angiospermic form can be distin- 

 guished. But they are modelled on the same plan, the female 

 and male sporophylls occupying the same relative position, and 

 the perianth is differentiated. These authors consider that some 

 important change in the mode of pollination caused the advent of 

 Angiosperms. 



Dealing with the ontogeny of Angiosperms in the paper already 

 cited, Bessey defines the difference in the development of the em- 

 bryo in monocotyledons and dicotyledons as follows : — 



"The only histological difference which is measurably constant is 



' " A Theoretical Origin of Endogens from Exogens by self-adaptation to an 

 Aquatic Habit." Journ. Linn. Sec, vol. xxix., 1892, p. 485. But see a more 

 recent paper, ibid. 



-Ibid., "Jour. Linn. Soc." 1907, p. 29. 

 VOL. L 17 



