SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 663 
diameter. The thin, smooth, cuticularised wall that alone persists has a character- 
istic bright yellow colour, and shows the tri-radiate marking. A number of spores 
lying free in a sporangium are shown in fig. 127. They have also been met with 
still grouped in tetrads. 
The Connection of the Parts Described and the Probable Habit of 
Asteroxylon. 
Before we can attempt to compare Asteroxylon with other plants it is necessary 
to briefly summarise and discuss the connection of the various parts described in the 
preceding pages, and in the light of this to see what picture can be formed of the 
general habit of the plant. Our knowledge is unfortunately based on abundant 
fragments only, and we lack the assistance of impressions which would give in whole 
or in part the general form. The position of some of the parts and their mutual 
connection can, however, be deduced on conclusive evidence, while the picture can be 
extended with more or less probability to include other parts, the connection 
of which is not actually established. 
The relation of the root-like branches of the rhizome to the peat, and the presence 
of stomata on the well-developed simple leaves, justify us in regarding Asteroxylon as 
a terrestrial plant which grew in and on the peaty soil. 
It is also clear that the plant of Asteroxylon had a leafless, dichotomously 
branched, subterranean rhizome, some of the branches of which passed by a gradual 
transition into aerial leafy shoots. The transition is marked by the appearance first 
of small scale-leaves and then of the larger foliage leaves. The internal structure 
shows a corresponding change from the simple stele of the rhizome without leaf- 
traces to the stele of the stem with a stellate xylem giving off leaf-traces. Other 
branches of the rhizome of varying degrees of slenderness have been shown by their 
relations to the peat to have taken the place physiologically of a root-system. There 
is, however, no evidence to justify us in drawing a morphological distinction between 
roots and rhizomes. 
The aerial shoots bore numerous, relatively small, simple leaves, each of which 
had a leaf-trace, though this did not extend into the free part of the leaf. The leaf- 
traces depart in more than one vertical series from each of the arms of the stellate 
xylem of the stem-stele. The aerial shoot must have formed a system of branches, 
maintaining a common plan of construction, but of successive degrees of slender- 
ness. The branching in most cases observed has been lateral, but dichotomy is known 
to occur both in large stems and at the base of a lateral branch. The facts on the 
whole appear to indicate erect shoot-systems springing from the subterranean leafless 
rhizomes. We have met with nothing pointing to a horizontal leafy stem giving oft' 
erect branch systems on the one hand and rhizomes with their root-like branches 
on the other. 
