LEAF-SHAPES AND MOISTURE. 
45 
there is a continual drip ; you will notice many leaves have 
brown spots caused by the standing of water upon them. An- 
other evil arising from this cause, especially in those countries 
where the sun shines powerfully between the showers, is that 
the drops of moisture act as so many burning-glasses, burning 
holes and brown spots in the leaf on which they rest. Drops or 
spheres of water are magnifying and burning-glasses. The little 
penny magnifying glasses hawked about the streets are simply 
spheres of water ; and we *sometimes read of fires which have 
originated through a globular water-bottle acting as a burning- 
glass when the sun shone upon it. 
Seeing, then, how hurtful it is to have the leaf surface 
covered continually with moisture, and yet that the foliage of 
Java remained unharmed, Stahl came to the very obvious con- 
clusion that there was some adaptation in the form or structure 
of the leaf which prevented this evil from affecting it. With 
this conviction in his mind he set to work to study the foliage 
of the Java forests, and what he tells us of this is equally true of 
the leaves of every other country in which rain and moisture 
have to be battled with. I will not attempt to set down here all 
the learned arguments and conclusions with which Stahl’s work 
is filled ; it is my object simply to ask you to notice what modi- 
fications of the leaf to enable it to cast off the water you can 
observe in our English plants, and I will state here some few 
cases of adaptation which may serve as guides in your own 
observations. 
If you walk round the garden shortly after sunset you 
will notice, if you look carefully, that many of the plants have 
their leaves arranged in a different direction from that which 
they occupied during the da}'. E.xamine, for instance, the 
bean plants ; during the day their leaflets were spread out hori- 
zontally so as to catch as many rays of sunlight as possible ; 
now in the evening they all droop in an almost vertical position. 
These movements of leaves after dusk are known as “ sleep- 
movements,” and occur in a great number of plants. Evidently 
the change from the horizontal to the vertical position must tend 
to throw off any water which may have collected on the surfaces 
during the day, and it also prevents the dew, which is so fre- 
quently formed in the cool of the night, from settling on them. 
Very similar to this is the sensitive plant of warmer climes, 
which, at the first drop of rain, folds up its tender leaflets, and 
thus prevents any further wetting. 
The formation of waxy layers is another adaptation which 
runs the moisture off like water from a duck’s back. Compare 
with this the fruits covered by their delicate “ bloom,” which is 
nothing more than a waxy film which serv'es the same end as in 
the leaf. Hairs also, which cover the surfaces of so many 
leaves, perform the same duty in another way. They act like 
blotting paper, spreading out the round drops into a thin layer, 
which evaporates at the first gleam of sunshine. Notice, in this 
respect, the leaves of the garden and greenhouse geraniums. 
