1918] SAM PSON—ABSCISSION ‘ 37 
layers in the petioles of the upper 3-5 pairs of leaves accompanying 
the development of the floral axis, these leaves remain on during 
the entire flowering and fruiting period. An investigation of the 
internal changes accompanying these 2 phenomena may throw 
some light upon abscission in general. 
Organic acids as a cause of leaf-fall 
WIESNER (17-21) cites three lines of experimental evidence as 
proof that the dissolution of the middle lamella is a result of the 
accumulation of organic acids in the aging leaves: (1) yellow leaves 
macerated and extracted with water when titrated were more acid 
than green leaves; (2) cuttings placed in 2.5 per cent oxalic acid 
dropped their leaves in a few days; (3) the exposed abscission 
surface of the petiole always gives an acid reaction to neutral red. 
Abscission in Coleus when examined from the point of view of 
this theory shows several facts in disagreement and not one in its 
favor. Cuttings placed in non-toxic concentrations of oxalic acid 
showed no acceleration of leaf-fall over that of cuttings in distilled 
water. Cuttings in 0.0002 N oxalic acid showed slight toxic effects. 
The immersed part of the stem and the tips of young leaves on 
axillary branches became brown in color. The concentration of 
acid used by WIESNER was 1500 times as great, but he fails to state 
what plants were used and whether toxic effects were produced. 
Cuttings of Coleus in 0.0016 N oxalic acid did not show an accelera- 
tion of leaf-fall, although the toxic effects were strongly pronounced. 
It was further found that the plants soon became adjusted to the 
oxalic acid. Plants started in 0.0002 N oxalic acid were transferred 
every third day to a concentration of acid double that of the 
previous concentration. This was continued until the plants were 
finally placed in 0.0512 N oxalic acid. There was no acceleration 
of leaf-fall during the entire period. At the end of the treatment 
the cells of the plant were found to be filled with starch. 
Similarly, potted plants infiltrated with non-toxic concentra- 
tions of oxalic acid showed no acceleration in leaf-fall. The plants 
were inverted under bell jars in vessels containing the various 
solutions, and the air was exhausted to 3cm. of mercury. The 
volume of solution entering the infiltrated plant was approximately 
