[nt son CERTAIN STRUCTURES IN THE PTERIDOPHYTES 409 
and so the lumina become available for the intrusion of the adjacent 
living cells. The vessels are thus blocked up with a living tissue, which 
in the first place prevents aeration, and then, owing to rapid decay, causes 
the death of the Palm. 
This concludes our observations on the nature of thylosez and it is 
now seen that the investigation of the groups of large, irregular cells 
ebserved in the bundles of mature stipes of Pteris aquilina has thrown 
considerable light on the vexed question of the meaning of the thyloses. 
For, it has been clearly proven, that thylose formation is essentially only 
a disposition on the part of the plant to restore a disturbed equilibrium, 
the activity ceasing as soon as the equilibrium is attained. It is no doubt 
for this reason that the thyloses are often so localised, blocking some 
portions of a passage completely (in transverse section) whilst others 
only in part, for all the parenchyma cells bordering on the canal do not 
possess the same degree of turgescence, and hence the equilibrium of 
some cells is disturbed less than that of others. Therefore some of the 
cells grow and divide more actively than others, whilst some may remain 
entirely quiescent. 
The thyloses, however, are only a secondary appearance, and it is 
the presence of a well-marked intercellular passage in the vascular 
bundles of the Ferns that is most significant, for it shows that this 
structure occurs in all the classes of vascular plants, from the Filices to 
the various Monocotyledons, originating in the same manner in all of 
them. Its general occurrence in the Ferns as a fully functioning canal 
only at the time when there is no other channel available for the ascent 
of the transpiration stream throws considerable light on its utility 
wherever it appears, whether it persists through the whole life history 
of the plant, or becomes blocked-up when its importance is greatly 
lessened. 
In conclusion, I wish to express my sincere thanks to Professor 
D. P. Penhallow for the interest he took in this research, and for the 
valuable suggestions he was ever ready to give when any difficulties were 
encountered by the writer. I must also thank him for the trouble he 
took in preparing the microphotographs illustrating this paper, and for 
all the facilities I was afforded in carrying on the work in the botanical 
laboratory. 
To Assistant Professor Miss C. M. Derick, also, I desire to express 
my appreciation for the interest she exhibited in the progress of the in- 
vestigation and for her many helpful suggestions. 
Sec. I1V., 1907. 26. 
