ORGANS OF FLOWERING PLANTS 19 
substanees that are essential to animal existence: and 
let us say at once that it is the chlorophyll alone that 
has the power of utilizing the sunlight which supplies 
the energy needed for this important work. Thus the 
chlorophyll is the one link between the world of lifeless: 
matter and living animals. 
Having now examined several simple leaves, let us 
see what peculiarities appear in the leaf of the bean. 
We observe first that the blade is not all in one piece, 
but consists of several leaflets. The midrib, which is 
really a continuation of the petiole, is quite free of the 
leaflet blades, while each leaflet has its own midrib, 
which branches like a feather to form the smaller 
ribs or veins. At the end of the main rib is a small 
loose end, This is really a much reduced leaflet, which, 
in many members of the bean family, as for instance 
the pea, is modified to form a tendril for elimbing. 
Just where the petiole joins the stem are two small 
flat outgrowths. These are called stipules and do the 
work of protecting the young leaf while still a tender 
bud. At the same time, being green, they do some of 
the work of leaves. 
Such permanent peculiarities as those we have noted 
are the result of changes that have taken place 
eradually in the course of ages, but there are also 
special variations that arise in the leaf when 
crowing under unusual conditions. Thus, when grow- 
ing in the dark, the petioles lengthen enormously in an 
effort to earry the blade out to the light, while 
the blade itself, since under such conditions it is 
a practically useless appendage, almost disappears. 
The plant responds to environment and expends its 
chief energy in the direction most likely to produce 
useful results. 
