216 
connection with an absorbing system (roots), conducting tissue 
for both water and elaborated products became a necessity; and 
lenin was brought into existence for mechanical support. Finally, 
in order to secure dispersal, a resting stage was intercalated in the 
the 
spore, which developed a cuticularized wall and was not only light 
but was capable of resisting heat, cold, and desiccation, and would 
alternating life-cycle. This stage was reduced to a single cel 
hence retain its vitality until it reached a suitable environment for 
germination. 
The earliest abundant representation of recognizable land plants 
that has been discovered occurs in rocks of Devonian age, although 
many students believe that the first invasion of the land must have 
been at a far earlier period. Among the rather numerous frag- 
ments that have been named, our interest centers on those plants 
which are more completely known. Among these, the first to be 
described and the one with the greatest range in time and space is 
Psilophyton, which was described by Dawson in 1859 and which 
is shown in the left hand corner of the transparency. 
Dawson's account was long regarded with great scepticism, and 
it was not until similar petrified material was described in 1917 
that the essential correctness of his work was recognized. 
Psilophyton was a plant ten to twelve inches tall, which thickly 
carpeted the Devonian bogs, especially those of the lower and 
middle Devonian times. The erect shoots were borne on a creep- 
ing rhizome clothed with absorbing hairs. The slender stems 
yranched and either smooth or covered with 
short spine-like outgrowths. Distally the tips of the branches 
were unfolded from crozier-like coils (circinate). Some of the 
— 
were dichotomously 
ultimate branches bore enormous sac-like sporangia. The stem 
had a central solid stele (axial cylinder of vascular tissue), and 
there were no leaves. In life the stems were green and performed 
the photo-synthetic function. This we know because stomata have 
been determined on their surface. Our knowledge of all these 
features indicates painstaking investigation of the carbonized 1m- 
pressions by students of paleobotany. 
The somewhat similar-looking Rhynia, shown next to Psilo- 
phyton, was undoubtedly related to it, and is much better known 
since it 1s preserved in a petrified condition in a silicified peat in 
