146 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9. 
runner in a stock bed of hostoniensis. Both the first two leaves were twice 
pinnate. This small plant was set apart and most carefully cherished. As 
proved by later developments, it was worth many times its weight in gold. 
Its third leaf must have been a disappointment, for it represented a return 
to the once pinnate form and may therefore be considered the original ex- 
ample of reversion. Thenceforward the original Piersoni plant and the 
thousands of progeny of this variety have always shown a mixture of leaf 
types: viz., typical twice pinnate, once pinnate, and intermediates on which 
entire, lobed, and divided pinnae may co-exist. Even a single pinna may 
show all three grades of division (PI. V, figs. 1-6). 
Not only is there variation in leaf division on a single plant at any given 
time, but there seems also to be seasonal variation or possibly variation 
according to different cultural conditions. In April, 1916, a group of about 
fifty stock plants of Piersoni was observed in the John Lewis Childs Floral 
Park greenhouses which were almost completely in a once-pinnate condi- 
tion. The plants were growing set out in rich soil in a well lighted bench. 
Most of them possessed only once pinnate leaves, but their identity was not 
possibly in doubt both because records had been kept of the planting and 
because of subsequent history. Three of these stock plants possessing only 
once pinnate leaves were selected for special observation with the thought 
that they might represent permanent reversion to a once pinnate condition. 
After being potted up at the Botanic Garden they were given good condi- 
tions of lighting, etc. The new leaves which followed were mainly of the 
twice pinnate type. Since then, all the plants of this origin kept at the 
Botanic Garden are predominantly twice pinnate, although cultural condi- 
tions have not been continuously at the optimum. 
The experience with the Childs plants just described bears out a con- 
clusion which Trevillian has reached after twenty years' experience with 
Nephrolepis varieties: viz., that during the less favorable cultural conditions 
of winter, all the division varieties tend to produce more reverted leaves 
than in the more favorable six months from April onward. It is a well 
established fact that all hostoniensis varieties show a recognized slowing up 
in the number of new leaves produced during winter, and apparently associ- 
ated with this is a tendency to produce leaves with less division. It may 
be concluded, then, that the maximum development of the special charac- 
teristics of any form depends upon an optimum of cultural conditions, and 
that it is therefore with Nephrolepis varieties partly a matter of season. 
The Childs plants above cited presented an extreme case of seasonal 
fluctuation. 
The condition of Piersoni with respect to the amount of leaf division 
may be described as a state of "fixed instability." Taken as a whole, the 
variety represents a distinct and definite saltation from hostoniensis, but 
the jump or variation was not all the way to a stable twice pinnate type al- 
though the predominant leaf form throughout the year and normally at any 
