VII. 



The Origin of Monocotyledonous Plants. 



IN the recently-issued number of the Linnean Society's Journal (vol. 

 xxix., p. 485) is a paper by the Rev. George Henslow, entitled 

 " A Theoretical Origin of Endogens from Exogens through Self- 

 adaptation to an Aquatic Habit." Endogens are, of course, mono- 

 cotyledons and exogens dicotyledons, and the author argues that the 

 former originated from the latter as the result of adaptation to an 

 aquatic habit. It may be of interest to see on what grounds he bases 

 this theory of the origin of one great group of angiospermous plants 

 from the other. 



Of geological evidence there is none, and in the paragraph headed 

 "Survivals" the calculation is not easy to follow. We read, "the 

 belief that endogens are of very early origin is supported by the fact 

 that so many orders of this class include very few genera. Thus, 

 according to the ' Genera Plantarum' of Bentham and Hooker, there 

 are 13 orders out of 166 which have only four or a less number of 

 genera, while two orders have six. Taken together, therefore, these 

 amount to nearly 8 per cent, of the whole class. It need hardly be 

 added that monotypic animals and plants, as well as those orders and 

 genera with but few members, are always regarded as survivals on 

 the principles of evolution and represent a lost ancestry." According to 

 the "Genera Plantarum" there are only 34 monocotyledonous orders, 

 15 of which, or 44 per cent., contain six genera or less. 165 orders 

 of dicotyledons and three of gymnosperms are recognised. That 

 there is some connection between an aquatic habit and endogenous 

 structures is evident from the proportion of aquatic orders ; for, 

 while only 7 per cent, of exogenous British orders are aquatic, the 

 proportion of endogenous orders is 53 per cent. ; and of the 224 

 exogenous orders given in Le Maout and Decaisne's " Analytical 

 Botany," nine may be regarded as aquatic, or 4 per cent. ; and of 

 55 endogenous orders, 18, or nearly 33 per cent. 



The numerous points of similarity in morphology, anatomy, and 

 histology between the two groups are adduced as evidence of 

 community of descent. Indications of an adaptive character are 

 also to be seen everywhere, and the author's object "is to show that 

 many of the characters common, more especially to the vegetative 

 organs of endogens, and regarded as points of affinity, are just those 



