410 Newcomhe—On the Cause and Conditions 
period of secondary growth and is represented by the pith of 
Juglans and of Sambucus. 
The immediate and apparent cause of the formation of 
cavity in such cases as those cited for primary growth, lies in 
the inability of the central mass of tissue to keep pace in 
growth with the more peripheral zones. If the peripheral 
zones continue to expand for a considerable period after the 
central mass has ceased growing, or if the peripheral part 
grows for a considerable time much more rapidly than the 
central part can extend, there will be formed a large central 
cavity during primary growth. If, however, the peripheral 
tissue ends its primary extension soon after that of the central 
mass, the cavity formed during primary growth will be small, 
but the cavity will subsequently continue to enlarge by the 
lysigenous process. 
The latter of the two cases just cited is that of some of the 
lower internodes of many plants, including Vida Fab a , Dahlia , 
and Ricinus , and passes insensibly into the condition in which 
the primary extension of the pith persists to the completion of 
primary extension in vascular zone and cortex. In the latter 
case the cavity-formation may begin soon after the ending of 
primary extension, or the pith may live on for weeks or years. 
The fact that in some plants, as pointed out for Urtica dioica 
and Dahlia , the cavity appears before or subsequently to the 
completion of primary growth according to the amount of 
primary extension of vascular zone and cortex relative to that 
of the pith, indicates that the life of the pith-cells is shortened 
by the tearing apart to which they are subjected in the one 
case, and indicates in the other case that the cells would not 
live much longer if not subjected to the tearing. But since, 
as has been pointed out by Kraus 1 , the stretching of cells due 
to turgor increases as they pass from the embryonal condition 
and decreases as they assume their permanent condition, and 
since the parenchyma, with thin walls of cellulose, such as 
generally makes up the pith, must contract when the force of 
1 Kraus, Die Gewebespannung des Stammes und ihre Folgen. Bot. Zeitung, 
1867, p. 105. 
