408 ROYAL SOCIETY OF CANADA 
general activity in the vessels of the whole plant. Again, the lumina, as 
has been pointed out, have not been available for a long time, and are 
only rendered available owing to the effect of the wound. 
Sachs, although he gives a wrong interpretation of the origin of 
thyloses, seems to have seen this relation of turgescence to the formation 
of thyloses in vessels, as the following will show (16, 581). “The whole 
process (of thylose formation) would be quite impossible if the tube of 
the vessel itself was filled with sap and turgescent, but as it is the vessel 
loses its sap, and the air contained in it is even rarefied, and thus the 
turgescence of the neighboring parenchyma cells must drive forwards 
the fine closmg membrane of the pits into the cavity of the vessel.” 
However, owing to his erroneous interpretations of the thyloses, he 
does not see that the reduction of pressure in the vessels causes the active 
parenchyma cells adjacent to them to grow into the lumina which be- 
come available for their extension. 
We will now consider one or two cases where thyloses appear in 
atrophied or diseased organs, in order to see if the cause of the formation: 
in these is the same as in the above instances. Strasburger (19, 411) 
furnishes us with the following example. “In a root of Monstera de- 
liciosa which was dead at its lower extremity, I found very beautiful 
Thyloses. In several places the vessels were completely blocked by the 
meeting of the thyloses. This ability to form thyloses in the vessels of 
the root is not limited to Monstera. I observed the same in root stumps 
of species of Philodendron.” 
In this case the water had ceased to circulate in the vessels, for the 
root had lost its power of absorption, its lower end being dead. Hence 
the pressure in the vessels was greatly diminished, and the adjacent 
parenchyma cells therefore availed themselves of the lumina to form 
thylosal tissues. 
Watt (24) gives an account of a disease which affects the Betel-Nut 
Palm, one of the symptoms of which is the blocking up of the vessels 
by thyloses. This disease occurs in periods of drought, when there is an 
excess of transpiration, and he says (24, 278): “ We must look, there- 
fore, to the deprivation of moisture from tissues that normally possess 
a large quantity of water for a possible explanation of the cause of the 
disease.” ; 
The thylose formation here further confirms the conclusions set 
forth above, for it occurs at a time when there is a great diminution of 
pressure in the vessels. This loss of pressure is due to the inadequate 
water-supply, the quantity of water available in the soil not being suffi- 
cient to supply the demands of the rapid evaporation. The water in the 
* vessels is thus given off, but no fresh supply rises up through the roots, 

