406 Newcomhe. — On the Cause and Conditions 
cellular spaces, there are all gradations shown by different 
plants. The essential difference of the two processes resides 
in the unlike development of the pith. In the first case the 
central and peripheral parts of the pith mature at about the 
same time, so that the schizogenous cleft is large and is 
followed by the rapid collapse of cells throughout. In the 
second case the central part of the pith matures first, while the 
peripheral part continues to grow, so that the first schizo- 
genous clefts, though small, are followed by the collapse of 
the few surrounding cells, and the cavity increases in size by 
the continued outward movement of the peripheral zone of 
pith and by the collapse of a few cells bounding the already 
existing cavity. Considered in toto , the schizogenous factor 
may be much larger in this latter case than in the former, 
though the initial clefts are much smaller in the latter. 
2. Lysigenous Cavity-formation subsequent to 
Primary Extension. 
Although with the cessation of primary extension the 
increase of tension between the pith and the more peripheral 
tissues, due to displacement of the peripheral zones, goes no 
farther, there are cases found in which cavities, though not 
appearing till after primary growth is complete, nevertheless 
begin schizogenously. In illustration of this statement may 
be mentioned the cavities in the pith of Althaea taurinensis , 
Silene viridiflora , and Eryngium planum. The cavities in 
these plants, due mostly to the collapse of tissue-elements, 
show themselves in their initial stage as small clefts between 
living cells. In the particular plants just named the cavities 
appear soon after the completion of primary growth ; the 
schizogenous clefts are, however, formed during primary 
growth, but the adjoining cells live for some time afterward, 
that is, till secondary growth has begun. 
Schizogenous clefts may, however, appear in the pith a long 
time after the surrounding zones of tissue have ceased to 
travel outward from the centre of the stem. We have only to 
