4 1 6 Newcombe . — On the Ctmse and Conditions 
growth, and about whose stems casts were placed before any 
cavity existed. In Urtica dioica the pith has thus been pre- 
served for three weeks within the cast after it had died outside 
the cast ; the possible period of its preservation was not deter- 
mined. Althaea taurinensis preserved its pith within the cast 
five weeks after its death outside the cast ; Lamium gar- 
ganicum , five weeks longer within the cast ; Dahlia variabilis , 
thirteen weeks longer within the cast ; Vicia Fab a, sixteen 
weeks longer within the cast. In only one of these plants 
was the experimentation followed far enough to determine 
the limit of this extension of the vitality of the pith, and this 
is purely individual, so that we cannot base any general 
conclusion upon it. This individual was Datdia variabilis , 
about whose stem a cast remained for six months. On 
examination there was found a very narrow strand of dead 
cells in the middle of the pith-cylinder. This period of six 
months, however, is as long as the ordinary life-period of this 
species ; and this fact, coupled with the case of Vicia which 
preserved its pith sixteen weeks and was then bearing fruit, 
makes it almost certain that with proper conditions the life of 
a normally early-dying pith may be made coextensive with 
that of the individual. 
With such an unusually long preservation of the pith there 
always goes a good growth of the stem, so that when the 
cast is removed its position is marked by a narrow neck 
connecting the much wider adjacent and normal parts. The 
same species that have been named as preserving their pith 
during such long periods have furnished examples of dying 
pith in shorter periods in individuals that made but poor 
growth after the application of the casts. 
In these cases of remarkably long-preserved pith there is 
apparently another factor, besides the prevention of de- 
structive tension, that is effective in giving the result, viz. 
the factor of regulation, in which the pith is called into use 
in the economy of the plant, probably to assist in transport 
through the narrow isthmus of the stem. 
Such a regulatory action would be similar to that recorded 
