542 PROF. D. T. MACDOUGAL ON THE GROWTH OF 
tissues will increase, as Jost has shown, in direct relation to the 
mass of the leaves, the fermentative processes will be controlled 
by the regulatory mechanism in like manner; but the increasing 
distance separating the leaf, the heightened difficulty in the way 
of drawing the reserve food from the more distant parts of the 
seed, and the constantly diminishing quantity of material available, 
due to its use and the intervention of destructive fermentations, 
will be such that the curves representing the needs of the leaf 
and the supply furnished will intersect before all of the material 
is exhausted. The point of intersection of these two lines marks 
the period beyond which development may not normally proceed 
without food-formation in the leaf. This period occurs at a time 
which is determined by the relative value of the factors entering 
into the formation of the two curves. In Zea it appears in 
10 to 12 days after germination and in Pheniv at a much later 
period. The above generalizations would also apply to specimens 
of Arisema and Solanum growing from tubers. By the removal 
of concurrent members from the former it has been shown that 
the point of intersection of the two curves may be set forward— 
a condition holding good also for Mimosa and Phaseolus in the 
darkness, but which has not been tested in light and in an air 
devoid of CO, In Trillium, Calla, and Lilium the intersection 
of the curves does not occur until the attainment of normal 
stature of the leaves. In Owalis and Isopyrum the intersection 
would not occur until some time after the formation of the 
leaves. In fact, if the tubers or rhizomes of these plants were 
deprived of all buds except one, this bud should be able to carry 
on development throughout an entire season or even longer at 
the cost of the disproportionately large amount of stored food 
available. The leaves of Justicia, Hibiscus, Fagus, and ZEsculus 
carry ou development at such a low rate that the chlorophyll 
areas of the unenclosed leaves are able to furnish a food-supply 
at all times equal to the needs of the enclosed organs, and hence 
the curves do not intersect, and the enclosed leaves are capable 
of perfect development and continued existence. 
In the light of my own experiments, it is difficult to accept the 
conclusion that the deterioration of leaves in an air free from CO, 
is due to the pathological effect of the disintegration of chlorophyll 
consequent upon its functional inactivity, as proposed by Jost (5). 
The ability of many growing and mature leaves to withstand 
the action of air free from CO, for extended periods, and the 
