40 8 New combe, — On the Cause arid Conditions 
thin diaphragm of dead, shrivelled cells with a rather wide 
cavity on each side. 
From this condition of the pith, in which some cells retain 
their position, we can pass on to that in Sambucus and 
Helianthus , in which all of the cells, though dead, retain their 
position, and the dead tissue has in it only the ordinary inter- 
cellular spaces. In these two plants, as is well known, the 
pith dies during the first season’s growth, but not till a wide 
zone of secondary formation has arisen. Details need not be 
given here of various other plants in which the death of the 
pith takes place one or more years after the first season, and 
it need only be mentioned that in such cases the pith some- 
times collapses, while at other times the cell-skeletons retain 
their primary position. The work of Gris 1 may be consulted 
for the age at which the pith of various plants dies. 
3. Lysigenous Cavity-formation either previous or 
SUBSEQUENT TO THE CESSATION OF PRIMARY GROWTH. 
It has been shown that nearly the same appearances accom- 
pany the formation of cavity during and subsequently to 
primary extension. In both stages of growth the cell-walls 
may be split apart, and intercellular clefts be formed, before 
any cells die ; in both, the collapse of cells may begin before 
the cells are separated by large clefts. It remains to be 
stated that there are plants of the same species in which, in 
corresponding internodes, the cavity is in one individual formed 
during primary growth, while in another it appears at a longer 
or shorter time after the cessation of primary growth. 
Urtica dioica and Dahlia variabilis verify the truth of the 
last statement. A strong plant of the former species exposes 
above the ground but five or six internodes before a cavity 
appears in the third internode, elongation being there incom- 
plete. Slender plants, however, may grow to a height of 
twelve internodes above the soil and still show no cavity 
Gris, Sur la moelle des plantes ligneuses. Ann. Sci. Nat. 5® ser. XIV. 
