Primitive A ngio sperms. 141 
Before proceeding to the examination of this evidence, it should be 
mentioned that the controversy is a very old one. Botanists have discussed 
the comparative antiquity of Monocotyledons and Dicotyledons for several 
generations. For a long time the prevalent opinion was that Mono- 
cotyledons were older than Dicotyledons — in other words, that Primitive 
Angiosperms resembled modern Monocotyledons in the characters in which 
they differ from Dicotyledons. 
Evidence in favour of this opinion was derived from three sources : 
the succession of fossil forms, the comparative anatomy of the stem, and 
the development of the embryo within the embryo-sac. 
The fossil argument was founded on a mistake. It is now generally 
acknowledged that Monocotyledons do not appear before Dicotyledons. 
On the contrary, they are found together in the earliest beds in which 
Angiosperms appear at all (Seward, 80 . a ; Zeiller, 91 , pp. 219-21). The 
arguments from stem anatomy and embryology will be considered under 
those heads. 
Stem-structure. 
As Primitive Angiosperms have not yet been identified in the fossil 
state, there is no direct evidence concerning their stem-structure. 
The features common to the stems of Monocotyledons and Dicoty- 
ledons are three. Their vascular skeleton is built up in the first instance 
of leaf-traces only. Each leaf-trace while young is collateral in structure. 
The xylem of each leaf-trace is endarch. We may assume that the stem 
of Primitive Angiosperms possessed these characters. 
The leaf-traces of the typical Dicotyledonous stem are arranged in 
a single circle. In each trace the xylem is internal and the phloem 
external, and they are divided from each other by a meristematic layer, 
the cambium. At first the cambium is not found outside the traces, but 
it soon forms a continuous cylinder within the stem, which produces 
secondary phloem on the outside and secondary xylem on the inside. The 
stem now in place of isolated leaf-traces displays a series of three concentric 
cylinders : centrifugal xylem, centripetal phloem, and meristematic cambium 
between them. 
In the great majority of Monocotyledons no thickening- ring of any 
kind is formed. The numerous leaf- traces are scattered over the transverse 
section of the stem with apparent irregularity. Their origin from leaves 
with a more or less definite phyllotaxy leads to a primary arrangement 
in concentric circles, and each leaf-trace as it enters the stem is orientated 
with internal xylem and external phloem. At this level it is collateral 
in structure, as in the leaf itself. There is no layer of meristematic tissue 
within the bundle, consequently secondary elements are never added 
to it. 
