of Lysigenous Cavity-formation. 419 
not enlarge ; it is confined by its thick-walled, outer zone, 
and by the xylem-ring which forms immediately on the 
removal of the cast. If the plant is sectioned a few weeks 
after the removal of the cast, the pith is found dead within 
the former limits of the cast as well as in other places. There 
is no question but that the removal of the cast induced the 
death of the pith, since numerous experiments have shown 
that when the cast is left longer, the pith lives longer. 
To my mind the only explanation of the facts recorded 
on the last few pages is this : The life of the pith is greatly 
prolonged by a regulatory process of the plant, the pith 
probably being used for transport through the narrow channel 
enclosed by the cast. If however the cast be removed so 
that the conducting tissue can be increased, the pith within 
the constriction dies, though it is neither stretched nor torn 
by the more peripheral zones. 
Summary. 
The results obtained from the foregoing record of experi- 
ments and argument may be comprised in the following 
statements regarding the formation of lysigenous cavities : — 
1. Whether the cavity shall appear during or subsequently 
to primary growth depends upon the cessation or retardation 
of extension in the tissue where the cavity appears, relatively to 
the extension in the more peripheral tissues. 
2. The initial cavity - formation in primary growth is 
always schizogenous, and may be schizogenous in secondary 
growth by the contraction of cells through their loss of 
turgor . 
3. In cavity -formation during primary growth there are 
always two factors , a schizogenous and a lysigenous . When 
the tissue in which the cavity is to be formed ceases to expand 
early in primary growth, the schizogenous factor is large and 
the lysigenous small. When the tissue in which the cavity is to 
be formed does not cease to expand till late in life , the schizo- 
genous factor is small and the lysigenous large. 
4. In cavity -formation during secondary growth the 
