243, 
isolated from other tracheze and contained, as before mentioned, 
only simple pores. Hereit is equally difficult to decide to which 
class this belongs. According to the wall-sculpture, it must be 
ranked with the thick-walled, closed, libriform cells, a classification 
which to us appears quite as unnatural as any of those based up- 
on the probable function of an element. 
Therefore, as no better method suggested itself, the generally 
conceded functions of the woody tissue were taken as a basis for 
division, viz: to furnish a means for the transport of water 
and to give strength and solidity to the stem. According 
to this idea, the different elements treated have naturally 
grouped themselves into two classes: those whose walls are so 
thin as to preclude the idea that they are of use as strength- 
ening-cells, and those which, according to their anatomical struc- 
ture and all the facts gained by experiment, may serve in both 
capacities, but more especially as strengthening-cells. 
By speaking of this as a method of division, it is not meant 
that it furnishes an exact criterion, but only that it avoids the 
necessity of one. Where the various facts concerning an element 
lead to the conclusion that its principal function is that of a water 
carrier, this element is referred to as tracheal; where the facts 
point to the opposite conclusion the, element is considered libri- 
form. According to this view the bordered pore is considered 
a peculiar organ of the water-transporting elements, and whenever 
it is found in the thick-walled cells its presence is supposed to 
indicate that these cells undertake both functions of the woody 
tissue. 
Since the completion of the foregoing pages, an article, 
“Ueber den Systematischen Werth der Holzstructur bei den Dicot- 
yledon,” has appeared, by Dr. Hans Solereder, Munich, in which 
this difference in the libriform tissue has been studied in a much 
larger number of families than the time and material for the 
present work has allowed. As the author only includes this 
among all the various other anatomical characteristics of the 
dicotyledonous woods, with no reference to any possible physi- 
ological meaning, he has, naturally, taken a different standard 
for the bordered pore, namely: the size of the border, and says 
wherever the border is larger than the “spfa/¢”’ the pore may be 
