82 THE ROYAL SOCIETY OF CANADA 
of pressure in the vessels.”” The thyloses were thus interpreted to be 
a function of growth dependent on purely physical causes. 
It is gratifying to know that Von Alten, the author of the most 
recent article on thyloses, has come to a similar conclusion in regard to 
the majority of cases of thylose formation (I, 21), as the following pas- 
sage will show: “In both cases I see the reason for thylose formation 
in physical factors, in opposition to those who assume chemical factors 
to be the cause.” Von Alten, however, divides thyloses into several 
classes, according to structure, function and origin, and, moreover, 
states that only those structures which grow into the lumina of vessels 
may be regarded as thyloses, while those which grow into open lumina 
are of a different nature. 
Our observations, however, have shown that all such structures, 
no matter whether they grow into lumina of vessels or into lumina of 
intercellular passages, are of the same nature and follow the same 
general laws. Again, thyloses do not arise in order to perform any 
special functions but merely as a result of certain conditions incidental 
to the growth and development of certain plants. Therefore no other 
function can be assigned to them, than the function assigned to the cells 
from which they originate. 
In the Coniferæ thyloses appear in the resin canals of certain 
forms several years after the formation of the passages, but it is signifi- 
cant that they arise only in those canals which are lined by thin-walled 
active cells and in passages of large lumen (Pinus), while they rarely 
appear in those canals lined by a thick-walled epithelium and of narrow 
lumen (Pseudotsuga, Larix). Thyloses are also found in the wood, 
especially in the heartwood of the root, of certain Conifer (8), and here 
also the general rules of thylose formation would apply, for a considera- 
tion of the features presented in their formation show that they originate 
when there is a diminution of pressure in lumina bordered by living 
cells. 
That the thylosal growths do not depend on the age of the tissues, 
but may appear at any stage of growth so long as living turgescent cells 
are bordering on lumina in which the pressure is less than the turgescence 
of the cell, is seen in the thylosal growths recorded in the first section 
of this paper in the pith of Pinus banksiana. 
These were formed at the most active period of growth of the plant 
by cells which, instead of remaining in contact with active elements 
like themselves, suddenly found themselves freed from this contact and 
opposed to merely air-filled lumina. They, therefore, in conformity 
with physical laws, extended into these lumina. Von Alten says that 
the thyloses of the root must be different from those of the stem in cases 
observed by him, because in the root they appear in the tissue which 
