[krrscH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 405 
Sachs (16, 581) has the following :—“ If the vessels, especially the 
wide ones, of Robinia, the Oak, Vine, and many other woods are exam- 
ined microscopically, when they have attained a certain age, they are 
found to be entirely filled with a parenchymatous tissue, which was 
observed and figured even by the first vegetable anatomist, Malpighi, 
though, of course, he had no idea of its origin. Not before recent re- 
searches, especially the careful labors of Rees (1868), were accomplished, 
was the remarkable origin of the thyloses rendered clear. They arise 
in fact by the very thin closing membranes of the bordered pits, at the 
spots where the vessels abut on soft parenchyma cells, becoming forced 
into the cavity of the vessel under the turgescence of the latter, and then 
beginning to grow vigorously. A club-shaped vesicle is thus formed, 
which, as it grows, undergoes cell divisions, and when such structures 
protrude from numerous pits, they fill up the cavity of the vessel and 
compress one another, and thus produce a parenchyma-like tissue.” 
Sachs is here evidently altogether wrong, for, like Bohm, he inter- 
prets the phenomenon in an impossible mamner. The closing membranes 
of the pits are only parts of the wall of the vessel, and at the time of 
thylose formation have lost the power of growth completely, being long 
past the formative stage. Sachs states that it is these membranes which 
are stretched and forced into the cavity of the vessel and there begin to 
divide. In order to be capable of growth and division, however, a nucleus 
at least must be present, which is not the case, since all the protoplasm 
has disappeared long before. If we accept Sachs’ interpretation we 
would thus have part of a cell wall, after being forced to bulge into the 
lumen of its cell, first forming a cross wall as in the case of the bud of 
an yeast plant, thus becoming cut off from the rest of the wall, and then 
beginning to divide and giving rise to a parenchymatous tissue. That 
this is impossible, is only too clear, for a cell wall cannot go on growing 
and dividing unless there is a nucleus with cytoplasm to cause this 
growth, the cell-wall itself being a dead substance. 
The explanation of their origin which is most in line with the re- 
corded observations, and which is confirmed by the nature of the phe- 
nomenon in Pteris and other ferns, is the following :—The parenchyma 
cells which grow into the pits and force the closing membranes into the 
cavity of the vessel are active, living cells, and as they continue their 
growth, they either rupture or disorganise the membrane, thus gaining 
access to the lumen of the vessel. Here they keep on growing and divid- 
ing as long as they do not encounter any resisting force, or until the 
food supply diminishes, and thus form a parenchymatous tissue. Thus 
these parenchymatous elements not only initiate the formation of thy- 
loses by forcing the closing membranes into the lumen of the vessel and 
