36 BOTANICAL GAZETTE } [yULY 
to become dry and then suddenly applying an excess of water. 
Under the first two treatments the leaves began to fall within 24 
hours. 
The rate of petiole fall after the amputation of the blade is 
shown in table I. At the beginning of the experiment each plant 
had 8 pairs of leaves and 1 blade of each pair was removed. The 
numbers refer to the total number of abscissed petioles at the cor- 
responding dates. 
TABLE I 
RATE OF PETIOLE FALL FOLLOWING AMPUTATION OF BLADES 
January February March 
Plant 
27 28 29 | 30 3r I 2 I 2 3 4 5 6 > Io 
Ber x I Be Ae Piel ee OT a Ee bee, ere Mag < 
Bo BoA ee ds ee OO ee ek ye eas 
4 Gee cel cbvwug hire loge ly vas le cea be Re atees of oS obec vec n 6 |. 
5 ee mee Wier alee heehee ce ie ea eae Ny 64, 
a gee Die obey ok Pecan vate. os pee ey ele a pat ae 6 61; Go 
* Blades amputated January 26; { blades amputated March 1. 
A microchemical analysis of the abscission layer in all these 
cases showed exactly the same changes in cellulose and pectic 
substances as noted under ordinary conditions of growth. Further- 
more, changes in oxidases, calcium in solution, and iron, to be . 
discussed later, were the same in all cases. These facts emphasize 
again the need of experimental investigation before accepting the 
turgor pressure theory of the cause of abscission. 
It is interesting to note that while the petioles usually absciss 
soon after amputation of the blade, there is one striking exception. 
If the blade is removed before the abscission layer is initiated, 
growth ceases throughout the entire petiole, the layer fails to 
develop, and the petiole is not dropped. The data in table I 
show that the 6 lowest petioles soon fall, but the upper 2 remain 
attached. This is an easy method of locating the period of forma- 
tion of the abscission layer. This extreme cessation of growth in 
the petiole induced by artificial means has some features in common 
with a retardation of growth and abscission formation in the petioles 
of the upper leaves under natural conditions of flowering and fruit- 
ing. Asa result of slowing up of growth and formation of abscission 
