4i 8 New com be, — On the Cause and Conditions 
showed within the casts a living pith nine weeks after it 
had died outside the casts ; Forsythia viridissima , Pterocarya 
fraxinifolia , and Juglans nigra , a pith living thirteen weeks 
longer within than outside the casts. 
If the objection be raised here that the pith lives because 
of the general vitality of the stem, the query might be put 
as to why it should thus live within the limits of the cast 
but die outside. Moreover there is still another series of 
experiments that give positive evidence on this point, and 
they will now be discussed. 
It will be remembered that Dahlia , Vicia Faba, and 
Melianthus major form a large cavity in their pith during 
primary growth. The last-named plant, when a cast is put 
about its stem before the cavity appears, remains solid over 
into secondary growth, but soon thereafter begins the forma- 
tion of periderm in the inner part of the cortex, and thus by 
the death of the external cells removes the resistance of the 
cast and forms secondary tissue rapidly. At the same time 
the pith dies within the cast. It has lived longer than it 
would normally, but though the tension upon it is not in- 
creased, it dies, probably because the plant does not need 
it longer to aid in transport through the constriction. What 
Melianthus does for itself has been artificially reproduced in 
the first two of the plants named above. Both of these had 
casts placed about their stems before a cavity was present, 
and the casts were allowed to remain for several weeks. 
During this time there is formed, in the outer part of the 
pith of these plants, a zone of thick- walled cells which 
become mechanical and form thus a bar to either the sub- 
sequent expansion or contraction of the central mass of 
thin-walled cells. The casts were now carefully removed 
without injury to the stem. The former location of the cast 
was marked by a deep constriction, and the pith within this 
isthmus was living and much less expanded than above and 
below, where it was dead. If secondary growth had not already 
begun, it now started vigorously, so that in a very few weeks 
the constriction was obliterated. The pith meanwhile does 
