1910] CROCKER, KNIGHT, ROBERTS—PEG OF CUCURBITS 335 
It is certainly evident, as all who have worked on this subject 
agree, that arching or “‘organ-form”’ stimulus, as NoLL termed it, 
leads to the shifting of the peg to the concave side of the hypocotyl, 
or, as it would be better termed, to the development of the peg 
on the concave flank of the hypocotyl. Both gravity and contact 
of coats aid in arch-production. Of these, as our results show, 
the latter is much more important. Contact of coats also greatly 
increases the size of the peg. Other questions that should be 
answered in this connection are whether or not gravity acts as a 
direct stimulus in the lateral placement of the peg, and whether 
or not it tends to increase the size of the peg on the flank exposed 
to it. 
As has beén repeatedly said, both DARwIN and NoLt assumed 
that gravity is a direct stimulus, not only in determining the 
existence of a peg but also its lateral placement. DARwin’s evi- 
dence for this was that a ringlike peg is found when the seedlings 
of Cucurbita ovifera were grown in a horizontal position on a slowly 
rotating clinostat. No ’s evidence for this conclusion included 
two other facts: (1) seedlings with the radicle pointing downward 
but deviated more than 8° 5 from the vertical gave pegs only on 
the lower side, and (2) seedlings turned over every few hours 
during development gave pegs ringlike or two-sided. We have 
shown that Notx’s deviation experiments do not at all accord with 
results obtained with all care to avoid the variation of more than 
one factor at a time. As for the clinostat and turning evidence, 
it is just what would be expected if the peg is an outgrowth approxi- 
mately equal on all sides, if sharp arching is avoided. Of course 
the clinostat and the turning from time to time prevent sharp arch- 
ing, and thereby give the natural ringlike or two-sided development. 
Further evidence as to whether gravity is a direct stimulus in 
determining the existence of a peg, its lateral placement, or its 
size, is found in the sections that follow. 
CENTRIFUGE CULTURES 
- A large number of cultures of the several varieties of cucurbits 
worked with were made on a centrifuge with a vertical axis. The 
acceleration varied from two gravities to eighteen gravities. The 
