of Lysigenous Cavity-formation. 407 
think of the pith-cells as diminishing the amount of their 
turgor while the outward pull of the more peripheral zone 
remains constant, to understand how, by the consequent 
inclination of the less turgid cells to decrease in size, the 
tension between such tissues is increased, and ensuing splits 
may follow before any cells die. In this manner are to be 
explained the clefts found in the pith of the lower internodes 
of many plants, which mark the beginning of cavity-formation 
a considerable period after primary extension has ended. 
In by far the largest number of plants in which lysigenous 
cavities appear in the pith after primary growth has ended, 
there is a collapse of cells before their separation by more 
than ordinary intercellular spaces. This is the case with the 
rhizome of Triticum repens , the cavity not being present for 
some weeks after the full diameter of the rhizome has been 
attained. The cells collapse and may split apart in so doing. 
The lower internodes of Lamium garganicum , Urtica dioica , 
Dahlia variabilis f Archangelica saliva , Vicia Fab a, Ricinus 
communis , and many other plants, form their cavities in the 
pith by the shrinking and collapse of cells, without showing 
previous separation of cells. 
In the foregoing examples there is a continuous cavity 
formed through each internode. Another group of plants in 
which the cavity is interrupted by diaphragms, is represented 
by Juglans , Pterocarya , and Forsythia , The formation of 
these diaphragms has been studied and described by Kassner 1 . 
The cells of the pith, none of which die for weeks after 
secondary growth has begun, begin to contract and separate 
in horizontal planes some distance from one another. Both 
the radial and longitudinal pull on the pith-cells is thus 
relieved, partially at least, so that the cells forming a hori- 
zontal plate midway between the planes where separation and 
collapse of cells has begun, are not pulled apart. Cells both 
above and below this plate of tissue are drawn nearer and 
nearer to it, till at last there remains only a comparatively 
Kassner, Ueber das Mark einiger Holzpflanzen. Inang. Diss. Breslau, 1884. 
