362 ae On the Genealogy of Plants. [June, 
exception of the medullary rays, entirely of large tubes, called 
tracheides, occupied with large prosenchymatous cells, which latter 
are nearly of uniform shape, while in the true Dicotyledons the 
tissue is in part parenchymous and the cells much more numer- 
ous and varied in form; moreover, the small circular areas 
enclosed between the walls of adjacent cells or źracheïdes are 
much more numerous and pronounced, especially in old tissue, 
in the Conifere than in the Exogens proper. 
There is still a third important respect in which the Gymno- 
sperms differ from the remaining Exogens in a marked manner. 
This is in the number of cotyledons, which is here usually more 
than two and sometimes as many as fifteen, while in true dicotyle- 
donous plants the number is uniformly two; only a very few 
exceptions having ever yet been found; as, for example, in Ranun- 
culus. ficaria, which usually has but one, and in some species of 
Phaseolus, which sometimes have a whorl of three. 
The objections above enumerated to the position of the Gym- 
nosperme in the prevailing system are quite independent of any 
recent facts pointing to their origin and derivation, and would be 
equally applicable under the old metaphorical conception of 
relationship. It is, therefore, all the more strange that it should 
ave survived so long and should have required the argument ) 
from descent to finally break it down. 
Evidence of this nature, however, is not now wanting, and it 
very plainly points to the direct filiation of the Gymnosperms 
upon the Cryptogams. This evidence concerns two important 
sets of characters, the woody tissue and the reproductive organs. 
As regards the former the close resemblance of the Cycadace@ to 
the arborescent ferns is very obvious from a glance at a cross 
section of each. If this character, therefore, possessed the 
importance which is claimed for it, it would be found more diffi- 
cult to pass from the Cycadacee to the Conifere than from the 
latter to the Dicotyledons. And we shall hereafter see that great 
liberty has been taken in thus grouping the Cycadacee and Con- 
ifere together. 
If we consider the Conifere alone there is one class of facts 
recently brought to light which possesses an unusual interest. 
The investigations of Prof. Williamson! have shown that the 
trunks of Lepidodendron, exhumed from the coal beds of England, 
1On the Organization of the fossil plants of the coal measures. Phil. Trans., 1872. 
Tt 
