SAP DENSITY AND FREEZING POINTS OF LEAVES. 115 
partly because the wind was from that direction, and partly, 
as will be shown later, from another cause. It was observed 
that the line of demarcation between the frozen and 
unfrozen portions of the leaf always followed the outline 
of the latter, so that, minus the frozen portion, the 
remainder had all the appearances of a true leaf. In the 
magnolias of this class the frozen margin has acted as a 
limiting band and the further growth of the leaf has 
caused an arching of the surface towards the upper side 
in Magnolia Lennei <, and towards the lower side in Mag- 
noha acuminata. The latter, while included in the next 
section, is perhaps as truly a member of this. It is some- 
what later than other members of the Magnoliaceae and 
had but few leaves exposed at the time of the frost. 
These were killed. The remainder, whose edges were more 
or less exposed in the opening bud, showed the effects 
mentioned above. That there exists a difference in the 
response of Acer and Magnolia to the same stimulus must 
be due to the difference in the maturity of the leaves, the 
former being probably more mature than the latter, and, 
consequently, incapable of growth recovery later, while in 
the latter, growth proceeded even after the leaf had been 
injured. Some new bud growth was induced, but since 
the effect was on trees that had already produced their 
spring crop of leaves, we may conclude that, in the absence 
of extensive new production of foliage, the productive 
ability of the remaining leaf surface would be much 
reduced, and possibly even to such an extent, that its 
effect would be perceptible in a difference in the size of 
the annual rings of the trees, or in a reduced activity in 
the foliar production of the following year, 7. e., in earliness 
and abundance of flowers and leaves, ete. 
SECTION ITI. 
We are forced to include this section, though the effect 
of the frost differs but in degree from the former division, 
mainly for the reason that we have here a method of 
