372 
POPULAR SCIENCE REVIEW. 
Botanic Garden by Dr. Anderson, after the terrific cyclone of 
October 1864. Several trees which were denuded of their foli- 
age by the wind produced new leaves, which, in place of falling 
off as they do under ordinary circumstances in winter, remained 
on the trees throughout that season. These trees did not, how- 
ever, flower in the following spring, as they would have done 
had there been no interruption to their ordinary course of 
proceedings. On the other hand, some others, although they 
produced a second crop of leaves after the cyclone, lost them 
again in the winter, and flowered as usual n the spring. These 
facts are very suggestive as to the relative interdependence 
of leaves and flowers ; a subject which we must not, however, 
dilate on in this place. 
Drought seems to have as potent an effect in bringing about 
the fall of the leaf as cold. In some of the Brazilian forests 
during the dry season the trees are as bare as with us in the 
depth of winter ; and a few summers since, when there was a 
long period of drought, the trees on the sandy dry soil about 
Bromley in Kent were as bare as in winter. Reasoning upon 
facts such as these, it is not unnatural to conclude that the fall of 
the leaf is in some way or other connected with evaporation or 
exhalation of fluid from their tissues. This, as is well known, 
takes place to a large extent, being regulated partly by external 
circumstances, partly by internal organisation ; and it has been 
proved experimentally by Lawes and others, that the so-called 
evergreens, as a rule, evaporate less in proportion to the extent 
of their leaf surface, than do ordinary plants ; and coupling this 
fact with the known effects of drought on deciduous trees, it is 
reasonable to suppose that the fall of the leaf is, in a measure 
at least, dependent on the evaporation from the surface. This 
explanation, however, is only partially satisfactory, for it will at 
once be seen that in most cases the leaves fall not at a period 
of drought, but in early winter, a time of comparative humidity, 
when heat or drought, at any rate, would not give rise to dis- 
proportionate exhalation. 
Adverting now to the external configuration of the leaf, let 
us see whether there may not be some mechanical cause for the 
separation of the leaf from the trunk. In most cases the leaf- 
stalk is attached by a broad base to the stem, so that when it 
falls off it leaves a scar more or less like a horseshoe in shape : 
on this scar may be seen the traces of the bundles of woody 
tissue, which passing from the trunk traverse the petiole or leaf- 
stalk on their way to the blade of the leaf, where they break up 
into the so-called veins. Now an observation of the shape of 
the scars, and of the disposition of the bundles of woody tissue 
in them, in different plants, throws some light on the causes of 
defoliation ; for instance, it is reasonable to surmise that where 
