148 
Journal of Agricultural Research 
Vol. xxvn, No. 3 
ACIDITY RELATIONS DURING THE PERIOD OF TRANSITION FROM 
THE VEGETATIVE TO THE FLOWERING CONDITION 
The data which have been presented in the preceding paragraphs show 
that decided change in the hydrogen-ion concentration of the cell sap is 
closely associated with change in the course of development of the plants 
as induced by increasing or decreasing the duration of the daily illumina¬ 
tion period. Thus, change from long to short days, which promptly ini¬ 
tiates flowering in such plants as cosmos and Biloxi soy beans, also 
results in marked change in the active acidity of the plant sap. The ques¬ 
tion arises at once as to the possible significance of the change in acidity. 
In this connection the time and extent of the change in acidity in relation 
to the time at which actual transition from one type of activity to another 
takes place is of special inter¬ 
est. It is obvious that abrupt 
change from long to short days 
acts very quickly in initiating 
the formation of flower prim- 
ordia in typical short - day 
plants, so that any causal fac¬ 
tor must be operative within a 
few days after the change in 
light exposure occurs. It has 
already been pointed out that 
decided change in acidity usu¬ 
ally becomes apparent about 
three to five days after the 
change in light conditions has 
been made. At this time there 
is a sudden and marked de¬ 
crease in acidity. This de¬ 
crease, however, is only tempo¬ 
rary and is promptly followed 
by abrupt increase in acidity 
to approximately the original level. This is followed by a further, slow 
increase during the development of the reproductive organs. 
These changes in acidity in Biloxi soy beans, as recorded in Table V 
(p. 13 7 ), are brought out more clearly in figure 10. It is believed that 
the sharp decline in acidity of the sap, occurring in this case 5 to 8 days 
after transfer to the io-hour day, definitely marks the actual transition 
from the vegetative to the flowering stage. It will be observed that a 
similar but somewhat less pronounced decrease in acidity occurred a few 
days later in the plants exposed to the natural decrease in length of day. 
These plants began flowering September 15 and it is known that after 
suitable change in length of day has occurred a minimum period of about 
25 days must elapse before the appearance of open blossoms. Hence, 
initiation of flower-bud formation must have been induced at about the 
time of the sharp decline in acidity. Since the natural decrease in length 
of day is gradual, it is to be expected that a critical point must occur at 
which there will be a condition of unstable equilibrium. A glance at 
figure 10 will disclose distinct evidence of such a wavering or “wobbling’' 
action, the final transition from vegetative to flowering condition appar- 
rently occurring about August 19, just 27 days before the appearance of 
open blossoms. 
Fig. 10.—Showing the temporary, sharp decrease in active 
acidity of the cell sap in Biloxi soy beans which results 
from suitable decrease in the daily illumination period 
(Table V). This change in active acidity apparently 
marks the transition from the vegetative to the repro¬ 
duction stage. The abrupt shortening of the daylight 
period produces a more decisive effect both in the tem¬ 
porary fall and in the subsequent rise in active acidity. 
The gradual, natural decrease in length of day seems 
to cause a less decisive wavering effect, probably indi¬ 
cating a rather delicate balance between the alterna¬ 
tive vegetative and reproductive forms of activity. 
