409 
of Lysigenous Cavity -formation. 
anywhere, though the lowest half-dozen internodes are fully 
elongated. In the average plant of Dahlia the cavity in the 
pith will be found present in all the internodes except the one 
nearest the ground before elongation has ended. Slender 
plants grow to a height of seven or more internodes, with the 
lowest five fully elongated, before a cavity appears. 
The histological differences attending the formation of such 
cavities in corresponding internodes before and subsequently 
to primary extension are not great. When the cavity appears 
while radial displacement of the vascular ring is progressing, 
the clefts between the cells become larger before cells collapse 
than in the other case, where primary extension has ended 
before the cavity forms. Moreover, in cross-section the cells 
of the pith are easily seen to be smaller in those slender stems 
in which the pith lives on into the period of secondary forma- 
tion, than in the individuals in which it dies earlier. There is 
also apparent in the vascular and cortical zones of the thick 
stems a greater tangential expansion relatively to the size of 
the pith than in the slender stems. In other words, the primary 
radial and tangential extension of cortex, vascular zone, and 
pith have, in the slender plants, more nearly coincided in time 
than in the thick ones. The length of the internodes, however, 
is as great in the slender as in the stronger plants. 
When the foregoing facts are properly arranged it will be 
seen, I believe, that the formation of cavity during primary 
extension is to be traced ultimately to the same cause as the 
formation of cavity or the death of the pith subsequently to 
the cessation of primary extension, that is, to the fact that the 
cells concerned have reached the stage where, without the pull 
of the more peripheral tissues, they would soon die. That, 
however, the life-period of such cells would be slightly 
prolonged did this forcible tearing not occur will appear from 
what follows. 
We have, then, a series in the formation of cavity, the one 
extreme of which falls in the period of primary extension and 
is represented by the axial tissue of the leaves of Allium Cepa 
and of the stem of Dahlia , while the other extreme falls in the 
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