5° BOTANICAL GAZETTE [JULY 
of abscission is a very rapid one or that the diffusion of calcium from 
cell to cell is a very slow one. This fact also explains why the addi- 
tion of calcium salts, already discussed, showed no specific effects 
on the rate of leaf-fall as the rate of diffusion of the calcium ions 
through the cells would again appear as a limiting factor. 
Extensive comparative investigations of abscission in the light 
of facts discovered in Coleus are still to be made. ‘Tison’s state- 
ment that in general the secondary membranes also are altered in 
abscission indicates that these processes may be rather general, 
particularly since the reports of HANNIG and Lioyp of a similar 
alteration in the abscission of floral organs. Investigation of the 
more fundamental factors underlying the ultimate chemical changes 
discussed in this paper must be made before general conclusions of 
the causes leading up to abscission can be drawn. The significance 
of the presence of an abundance of nitrates in the abscission layer 
at the time of abscission is uncertain. Their ability to affect the 
water holding capacity of colloids and similar effects of other ions 
which are changing in concentration in this region may influence 
the permeability of the cell membranes of these cells, a question 
that has not yet been touched upon experimentally. 
Conclusion 
Abscission of leaves in Coleus Blume? is a result of the conversion 
of cellulose into pectose, which is further transformed to pectin and 
pectic acid, leading to the formation of an excess amount of pectic 
acid over that of the available calcium sufficient to maintain the 
solidity of the middle lamella of the cell walls of the abscission layer. 
These processes are possibly initiated and probably accelerated by 
the presence of oxidases and ferric ions, both of which accumulate 
in the abscission layer. 
Microchemical methods employed 
In the microchemical study color reactions were used for orienta- 
tion. ‘These were followed by specific chemical reactions and solu- 
bility tests. A brief outline of the tests made for each substance 
follows. Details of these reactions may be found in recent micro- 
chemical texts. 
