S3 
Commencing with a statement of Brogniart’s dictum, 
that Lepidodendron was a cryptogam and Sigillaria a 
gymnosperm, and showing by a sketch the appearance 
which led Brogniart to found his distinction between 
them, viz., the appearance in the latter of an exogenous 
vascular zone externally to the non-exogenous vascular 
cylinder which he found in the former genus, Professor 
Williamson proceeded to show that the presence and 
size of the ring of exogenous growth in the stem was 
merely a question of the age of the plant examined. 
Further, that the markings on the outside of the stem, sup- 
posed to be diagonal in Lepidodendron and vertical in Sigil- 
laria, probably depended upon differences which were not 
even generic, but which might have occurred in the same 
plant in different stages of growth, the Lepido dendroid con- 
dition being the younger one. 
Turning to Stigmaria, Professor Williamson showed 
that they possessed two striking peculiarities — 1st, they 
branched and subbranched dichotomously. Such branches, 
however, had no share in the work of absorbing nutriment 
from the soil. This function was performed by large but 
delicate rootlets given off abundantly from the surface 
of each root branch. 2ndly, these rootlets possessed a 
single bundle of small , vessels running longitudinally 
through the cortical investment. In each rootlet this 
vascular bundle commenced its growth as a single vessel, 
developed eccentrically within an innermost cylinder of 
cortical cells, and additional vessels were added to the bundle 
centripetally as the rootlet advanced in age and size until 
it developed into an eccentric cluster of 40 or 50 vessels. 
This peculiar arrangement is only found amongst living 
plants in the Ophioglossums and Lycopods, in the latter of 
which alone does the dichotomous branching of the roots 
occur. It would appear, therefore, that the two features, 
viz. the peculiar growth and structure of the rootlet and the 
