44 THE REV. GEORGE HENSLOW'ON THE ORIGIN OF THE 
The relative Ages of the classes Dicotyledons and Monocotyledons.—1f any thing as to 
the relative ages of these classes can be argued from the foregoing facts, it would seem 
that Monocotyledons are of greater antiquity than at least angiospermous Dicotyledons, 
in that they furnish differentiated stages of leaf-arrangement issuing from an arrested 
condition of the embryo—that is, on the assumption of whorled leaves having preceded 
opposite, and opposite alternate, and that the single cotyledon of a monocotyledonous 
embryo has resulted from the arrest of another*. Тһе fragmentary character of many 
monocotyledonous groups, as witnessed by a greater proportion of isolated orders and 
genera t than in angiospermous Dicotyledons, as well as the very general prevalence of 
albumen (excepting water-plants), i. e. a retention of an embryonic character, may be 
brought forward as reminders that other facts corroborate this idea ; while, as far as the 
scanty evidence of geology can take us, Monocotyledons seem to have been represented 
in the primary epoch (though some paleontologists contest this), yet no instance of an 
angiospermous Dicotyledon has yet been forthcoming from the same period. 
Explanation of the Scheme (fig. 3).—In this scheme are represented the relative 
positions of the leaves of the ordinary phyllotaxis. The figures connected by double 
lines are those which give rise to Ute fractions 
8; & ў, 1% С 21,4 2 Г» HE 
* In suggesting this process of reduction from whorled leaves as the origin of opposite in Dicotyledonous embryos, 
and of a single eotyledon in Monocotyledons, I may seem to differ somewhat from Mr. Herbert Spencer’s views 
expressed in his * Principles of Biology,’ vol. ii. p. 67, where he conceives of both Dicotyledons and Monocotyledons 
as diverging from a common ancestral type, the former having the first two folial or thalline organs flattened out in 
virtue of the requirement of strength by a development of the midrib, which therefore does not necessitate their 
being rolled into the form of a cylinder to acquire strength, as in Monocotyledons ; and, secondly, to use his own 
words, “if we assume, as the truths of embryology entitle us to do, an increasing tendency towards anticipation in 
the development of subsequent fronds—if we assume that here, as in other cases, structures which were originally 
produced in succession will, if the nutrition allows and no mechanical dependence hinders, come to be produced 
simultaneously, there is nothing to prevent Га passage from an , alternate to an opposite condition in the develop- 
ment of the first two thalline or folial expansions]. - 
If this ** assumption ” simply means * arrest" of the internode of the second frond, he is probably right, as it 
appears obvious that an alternation is aequired solely by the development of an internode. But the process of such 
internodal development appears rather'to be due to the presence of extra nourishment and rapid growth, than the 
“reverse, as seems to be implied by Mr. Spencer ; for his words intimate that nutrition must determine the opposition 
rather than the alternation. But I cannot help thinking that neither nutrition nor the want of it is the vera causa. 
They may determine more or less the growth but not the development, It is to me inconceivable how mere physical 
causes could have produced, for example, the fundamental distinction between an exorhizal dicotyledonous axial root 
and the endorhizal character of monocotyledonous adventitious roots—a feature which Mr. Spencer does not appear to 
_ have considered, and which is really due to the arrest of the primary or axial root in the latter, and which is there- 
fore correlated with an arrested cotyledon ; and, it may be added, an arrested condition is the presence of опен 
endosperm, so general in all terrestrial Monocotyledons. 
Hence I eonclude that an alternate or spiral arrangement in Dicotyledons results from the development of inter- 
nodes, but in Monoeotyledons in consequence of the arrest of one cotyledon. 
+ I find from a calculation of the orders and genera given in Le Maout and Decaisne’s * Descriptive and Analytical 
Botany,’ edited by Dr. Hooker, that of Monocotyledons about 60 per cent. of the orders comprising that subkingdom 
have less than 6 genera, and nearly 33 per cent. have less than 3 genera; whereas of Monochlamydeous exogens, 
omitting Gymnosperms, there are about 40 per cent. of orders with less than 6 genera, and 26 per cent. with less 
than 3. Lastly, of Dichlamydeous exogens, there are no more than 28 per cent. of orders with less than 6 genera and 
about 15 per cent. with less than 3. : 
