of Lysigenous Cavity formation. 413 
like manner. The stem here did not increase in diameter 
after the cast was applied, but the enclosed internode, when 
examined, was but one-third the length of adjacent internodes. 
The small cells that are usually destroyed when the canal 
is formed were living and intact about the annular vessel. 
The explanation of the prolongation of the life-period of the 
tissue in the foregoing five cases otherwise than as the result 
of relieving the tension, seems to me improbable. In the one 
case only of the leaves of Allium Cepa in gypsum-casts was 
there any appreciable constriction of the part in which the 
cells in question were preserved. When the constriction is 
considerable, it is conceivable that the cells might by regu- 
latory means be kept alive for purposes of transport. But 
I have produced in several species of plants, by means of 
gypsum-casts, segments of stems with one-fourth the area in 
cross-section of the same stem above and below the cast, and 
all without an apparent effect on the transpiration or vitality 
of the plants. Such plants have been obliged, therefore, to 
carry their transpiration-current through a channel one-fourth 
as great as in other parts of the stem. It is thus demonstrated 
that, for ordinary transpiration, these plants do not need their 
full amount of xylem and conducting tissue. Hence it is 
pretty certain that the central cells in the segments of the 
Onion-leaves encased in gypsum were not kept alive for needs 
of transport, since the conducting channel in them was not 
greatly narrowed by the cast. 
Finally, the cases of Urtica dioica and Dahlia variabilis may 
be again cited as furnishing evidence for the varying duration 
of the life of the pith according to the tearing action of the 
peripheral zones upon it. It will be remembered that in these 
two species the cavity appears in thick stems during primary 
growth, but in slender stems during secondary growth. 
The life-period of cells cannot, however, be indefinitely 
prolonged by averting destruction due to tension : in fact, the 
destruction of cells by tension acting between tissues is an 
indication that such cells are near the end of their life-period. 
As already stated, etiolated leaves from the bulb of Allium 
