70 
PLANT MORPHOLOGY. 
in these several plants, but in a definite manner in each. 
In the elm, the distribution of the leaves is such that 
the third leaf is directly above the first; in the beech, 
the fourth leaf is above the first, and in the oak, the 
sixth leaf is above the first. If these leaves are con- 
nected in the order of their arrangement, it will be 
seen that they describe a spiral in their development, 
and it will also be found that one or more turns are 
made between the superimposed leaves. Furthermore, 
it will be found that this arrangement constitutes a 
mathematical series which may be expressed in degrees, 
or the parts of a circle that the leaves are from each 
other, this measure being known as divergence ; or by 
the number of perpendicular rows of leaves on the 
stem which are known as orthostichies. 
The following may serve to illustrate the terms used : 
Divergence. Orthostichies. 
Degrees. Parts of a circle. 
Elm 180 J Distichous. 
Beech .... 120 ^ Tristichous. 
Oak 144 | Oentastichous. 
If we examine the fractions used, we will find that 
the numerator indicates the number of turns around 
the stem before encountering a superimposed leaf, and 
that the denominator indicates the number of leaves 
found ; the latter also expresses the number of ortho- 
stichies. On adding the numerators and denominators 
of any two successive fractions, a fraction is obtained 
which expresses the next highest arrangement, as 
2 + J = f • 
In quite a number of plants two leaves arise at the 
nodes, as in the Labiatre. These are invariably situ- 
ated opposite each other on the stem, and the succes- 
sive pairs alternate with one another, forming the 
decussate arrangement of leaves. 
