[KIRSCH] ORIGIN AND DEVELOPMENT OF RESIN CANALS 81 
medullary rays and in vertical strands, while in the former the parenchy- 
matous tissue was of limited extent. The absence of ducts in the 
parenchyma of the woody Dicotyledons is due to the presence of the 
vessels which release any tensions established in the tissue by their very 
rapid growth. 
In abnormal cases, however, such as the proliferation of the medul- 
lary rays in Betula papyrifera, where large aggregates of wood paren- 
chyma are present without containing any vessels, a distinct tendency 
to canal formation is seen. This point is well illustrated in an observa- 
tion recorded by Wigand (88, 123). Ina tree of Prunus avium, which 
is normally distinguished by the small amount of parenchyma in the 
wood, he observed tangential rows of cysts present in the immediate 
vicinity of the wound. “Moreover, while wood parenchyma is ordin- 
arily absent, in cases of this sort strands of wood parenchyma which 
traverse the wood in a vertical direction were found. These cells agree 
on the whole with the cells of the medullary ray, both in structure and 
contents and also as regards reactions with certain reagents. In these 
strands of cells gum reservoirs are formed. The passages do not trans- 
verse the whole length of the strand but are broken here and there by 
wood parenchyma cells, so that there is instead of a continuous passage, 
a series of cysts which are roundish or fairly long.” Thus the formation 
of resin passages is paralleled here, since the conditions are of a similar 
nature, the excretion only being of a different kind. 
II. Thyloses. 
The phenomenon of thyloses was fully treated in a previous paper 
by the writer, and the observation made in connection with the thyloses 
of the Coniferæ confirmed the conclusions arrived at in the case of the 
Pteridophyta. In the consideration of the thylosal growths which 
appear in the canals originating by the degeneration of the protoxylem 
in the Pteridophyta, it was observed that (15, 403), “ During the time 
that the sap ascends only by way of the intercellular canal, however, 
the pressure exerted by it on the wood parenchyma cells greatly exceeds 
the pressure arising from their highly turgid condition and prevents 
them from extending into the cavity. When this force is diminished, 
their turgidity exceeds the pressure exerted against them, and they 
begin to grow into the canal, and, since a large quantity of nutritive 
substances is available, they divide rapidly.” Further on, in extending 
these observations to the Betel-Nut Palm, the following statement was 
made: “The thylose formation here further confirms the conclusivns 
set forth above, for it occurs at a time when there is a great diminution 
Sec. IV,, 1911: +6, 
