OBSERVAT/ONS FOR YOUNG BOTANISTS 225 
Monocotyledons — Embryo with one cotyledon ; Axial root 
arrested ; Stem with woody cords isolated ; Leaves with parallel 
venation ; Floral whorls in threes. 
A study of these differences reveals the fact that the latter are 
very often to be found in aquatic plants of the former ; and the 
accumulation of coincidences is so great as to warrant the belief 
that monocotyledons have descended from such plants occurring 
among dicotyledonous families.* 
To mention a few e.xamples — The Cotyledon. Several facts 
point to an arrest of one of two cotyledons in monocotyledons. 
Thus asparagus has a rudimentary cotyledon besides the normal 
one. 
On the other hand, the water-chesnut {Trapa natans) and 
Ranunculus Ficaria among dicotyledons have only one. 
The axial or tap root is developed in ordinary land plants of 
dicotyledons, as in the carrot, parsnep, &c. : but it is arrested 
in the water-crowfoot and in the members of the water-lily 
family : just as it is in all monocotyledons, or else is verj' transi- 
tory, as is the date. 
The stems, as of an oak, compared with that of a palm, show 
marked differences. The former gave rise to the terms 
Exogens, or “growing on the outside,” i.e., of the previous 
year’s wood ; the latter, Endogens, “ growing from within.” 
This, however, rose from a mistaken notion, and is really 
inapplicable. t 
In exogenous, woody stems the woody or fibro-vascular 
cords are arranged in circles, forming a dense cylinder ; but in 
endogens they are scattered about. 
In the llov/er-stem of a daffodil they will be found quite regu- 
larly disposed, while in Nelumbium of the Nymphccacea the circles 
are separated, and show a tendency to be “dislocated,” as in 
palms. 
The parallel venation of the leaf of a monocotyledon is well 
seen in aquatic plants of this class,'for the “ ribs ” issue out of 
a collection of isolated cords of the stem, and simply run side 
by side up the long narrow “ leaf.” Moreover, if the leaf reach 
the surface, as in the arrow-head (Sagittaria), it then forms a 
blade at the summit, showing that the linear part is really com- 
parable with a leaf-stalk or petiole, and so might be called a 
phyllode. 
Lastly, numerous monocotyledons are now land plants. They 
have doubtless arisen from aquatic monocotyledons, as they 
have retained all the “aquatic features,” though now re-adapted 
to a terrestrial existence. 
The above notes are given to show the directions in which 
* A full account of this theory will be found in a paper by the present writer — 
Joiirn. Linn. Soc., vol. xxix., p. 485. 
t The word is retained as convenient for classificatorj’ purposes ; but the 
meaning must be dropped. 
