[kirscH] (CERTAIN STRUCTURES IN THE PTERIDOPHYTES 407 
clear that there is a great reduction in pressure in the lumina in every 
case where thyloses appear. 
In the case of Equisetum Telmateja mentioned above, the stem wa: 
decapitated and hence the force of the transpiration stream was greatly 
diminished or altogether destroyed. The stem, however, was kept under 
water, and so the parenchymatous elements surrounding the canals would 
retain their turgescence for a long time. The intercellular passages on 
the contrary, lost their fluid, as was evident from the drops of water 
that were observed to come out of them; and the equilibrium between 
the pressure in their lumens and that in the lumina of the surrounding 
parenchyma cells was therefore destroyed. This is exactly parallel to 
what occurs in Pteris aquilina when the intercellular passages begin to 
lose their function, and what occurs in them naturally occurs here also. 
This, of course, affords a rational explanation for the thylose formation, 
the mere stimulation due to the cutting of the vessels not being sufficient 
in my estimation, to induce such widespread tissue formation. 
The case of Tradescantia fluminensis is identical, for here also the 
continuity of the vessels wes destroyed, thus causing the diminution and 
loss in the vessels of the pressure due to the movement of the transpira- 
tion stream. The living cells bordering on the passage, however, do not 
lose their turgescence, and so naturally grow into the cavity, which 
has become available on account of diminished pressure. 
In both the above cases the passage offers the least resistance to the 
extension of the parenchyma cells, for no pit with a closing membrane 
has to be traversed before the lumen is reached as in the case of xylem’ 
vessels, and hence the activity becomes localised at this point. The action 
of wounding does not produce the thyloses directly, owing to stimulation, 
but indirectly, owing to the interference with the upward movement of 
the transpiration stream. When viewed in this light it is not at all 
strange that the phenomenon extends a great distance, for the effect is 
practically uniform all along the passages. 
Pfeffer, in discussing the effect of traumatic lesions has the follow- 
ing (12, 134) :—“ Special stimuli are, however, often involved, for in- 
juries may induce growth in single cells or tissues, which were quiescent 
in the adult plant, although no mechanical resistance was offered to their 
growth. This applies to the formation of thyloses, which only appear 
when the wood has attained a certain age, or when an injury acts as a 
stimulus to their formation, although the open lumina of the tracheæ 
have been available for a long time previously.” This is seen to be 
wrong in the light of what has been shown to be the most probable cause 
of thylose formation, for the stimulation due to the wound would only 
affect a limited area adjacent to it, and would not be enough to cause a 
