Facts relating to the Structure of Seedlings . 259 
is a striking inconsistency in the behaviour of the cotyledonary bundles 
in the transition region. 1 
In attempting to determine the main question — primitive or reduced — 
the investigator naturally turns to the fossil plants to see what evidence 
they afford. Unfortunately, in the case of seedling-structure such evidence 
is practically nil ; in fact it obtains only in one plant, Bennettites , which 
approximates so closely to the living Cycads that it affords practically 
no aid in the present connexion. Incidentally, it may be remarked that the 
Coniferae are of very great antiquity, and go back at least as far as the 
Permian, whereas the Cycadaceae first appear in mesozoic times. 
We are, therefore, driven to the existing Pteridophytes, which is not 
altogether satisfactory, since they may not follow in many of the details of 
their structure the organization of their ancestors and of the other phyla to 
which their ancestors may have given origin. The further objection may 
be raised against the comparison of, say, Gymnosperms and Pteridophytes 
on account of the differences due to the seed habit. 
In the modern Pteridophytes certain features, with regard to the 
structure of the sporelings, stand out with remarkable clearness. 
1. The primary root is almost invariably diarch. 
2. The leaf-trace of each of the first two leaves of the sporeling is 
a single strand. 
3. These bundles show no signs of bifurcation such as obtains in 
so many of the higher plants. 
4. There is no hypocotyl in the sense of this structure in the higher 
plants. 
Considering these features ; with regard to the root it is not at all 
remarkable that the organization of the primary root of a Fern and of 
a higher plant are so similar, for the radicle is one of the most conservative 
members of the plant, since its functions are definite, and there is no reason 
to suppose that the conditions of the existence of roots have altered to any 
appreciable degree through the ages, so that a variety of structure has not 
been called into being to meet varying requirements. 2 
It therefore does not appear unreasonable to suppose that a diarch 
root-structure is primitive : amongst the Angiosperms such an organization 
is extremely common, and with regard to the Gymnosperms diarchy, as far 
as our observations go, is by far the most common. This is shown in the 
following table, which gives the number and percentage of species of Gymno- 
sperms in which the number of root-poles was constant within the limits 
indicated in the first column. 
1 Hill, T. G., and de Fraine, E. : Gymnosperms, Pt. III. Ann. Bot., xxiii, 1909, p. 456. 
2 In this connexion primary root-structure only is considered. 
