404 ROYAL SOCIETY OF CANADA 
the importance of the canal as a conductive passage is not greatly less- 
ened on the completion of the secondary xylem, and the pressure exerted 
on the living cells which border it is not diminished to any considerable 
extent. As soon as this pressure is greatly lessened, however, thyloses 
make their appearance. This, I believe, is the primary cause of the 
abnormal thylose formation noted above in the cases of Equisetum Tel- 
mateja, Tradescantia fluminensis, and Nymphæa alba. To prove this, 
however, it will be necessary to go more fully into the subject of thylose 
formation, and to see what views are held in regard to its origin and 
significance, 
The first recorded observation of thyloses is that of Malpighi (7, 9), 
who did not attempt any explanation of their significance. 
Mohl (10), in the Ray Society’s volume for 1849, speaks of them 
as follows:—“I think that I am not wrong in assuming that they are 
produced by a protruding expansion (a kind of hernia) of the adjacent 
cell, which penetrates the pore, and either tears through or causes the 
absorption of the primary membrane of the vessel.” 
Bohm (1867) (1) on the contrary, held that the thyloses do not 
originate by the bulging out of the cells surrounding the ducts, but by 
the accumulation of plasma between the lamellæ of the walls of the 
vessels whose innermost layers grow out as the membranes of the 
“ Thyllen” cells. As will be shown below, this view is altogether er- 
roneous. 
Rees (1868) (14) holds the following view: “Each young thylle 
makes its appearance as a bulging of a wood parenchyma or me 
ray cell forced through a pore in the vessels.” 
De Bary (3, 170) describes their occurrence as follows :—“ They 
may arise where a Trachea borders on parenchymatous cells, and in fact 
from those cells themselves, which grow into it. A small part of the 
merbrane of a parenchymatous cell adjoining an unthickened point on 
the wall of a Trachea (as a rule a pit) grows to an excrescence protrud- 
ing into the cavity of the latter... and finally cuts itself off as a special 
cell from the rest of the cavity of the cell which produced it by means 
of a division-wall, formed at its point of entrance into the Trachea... 
This often happens to such an extent that the tube is entirely filled by 
thyloses flattened into polyhedral form by reciprocal pressure. Further, 
a multiplication of them by division has been observed in many cases.” 
He goes on to give an account of the forms in which they have been 
observed (Mono-and Dicotyledons) and concludes as follows :—“ These 
facts may afford starting-points for the enquiry into the still unknown 
causes of the formation of thyloses which cannot be further noticed 
here.” 

