of Vascular Strands in Angiopteris evecta . 399 
then it is best to enforce the distinction. In any event, to 
render its inclusion or exclusion contingent on the differentia- 
tion of a zone (endodermis) notoriously uncertain, and incon- 
stant even in nearly related forms, appears to us as a gratuitous 
introduction of a confusing element into a subject already 
sufficiently provided with artificial difficulties. 
It would appear to be probable that no right understanding 
of a difficult vascular structure is possible apart from a study 
of its ontogenetic development. Only in this way can the 
complex arrangements, such as may occur for example in 
the stems of many lianes, be understood and referred to the 
relatively simple condition typical of the great majority of 
dicotyledons. 
But although we are unable to accept, at least in their 
entirety, the views held in many quarters as to the morpho- 
logical relations assumed to subsist between the various tissues, 
we are far from underrating the value of evidence derived 
from a study of comparative anatomy in dealing with questions 
of affinities. On the contrary, we are completely in accord 
with those who regard such evidence, when properly sifted 
and checked, as of great value ; and it is largely on this very 
account that we have felt bound to state our grounds for 
diverging from the opinions entertained by many others as 
to the importance of the quasi-morphological relationships 
of the various tissue-systems to one another. But, as with 
all characters, the value of those which can only be distin- 
guished by the aid of a microscope requires to be appreciated 
with caution and applied with care. Perhaps, indeed, anato- 
mical characters need this even more than most others, 
on account of the relative ease with which so many of them 
are apparently susceptible to alteration or suppression. 
To return to the structure of the Marattiaceae, we may 
summarize our results as follows. The stem of Angiopteris , 
and that of Marattia is in close agreement with it in all 
essential respects, contains in the young plant a single solid 
protostele. In the centre of the xylem, certain coil-rows 
cease to differentiate as tracheids, but proceed to give rise 
