28 - ELEMENTARY BOTANY 
times they resemble narrow foliage leaves, arranged in whorls 
and completely enveloping the flower, as in daisy. The term 
involucre (Lat. involucrum, a wrapper) is applied to bracts which 
thus surround the flower. In hedge parsley, the bracts forming 
the involucre are thin, narrow, and green ; In the oak, the invo- 
lucre is cup-like, forming the well known acorn cup. In wood 
anemone, the bracts of the involucre are easily mistaken for 
foliage leaves; but the true foliage leaves are borne by theunder- 
ground stem, and the leaves on the flowering stem are bracts. 
Foliage If foliage leaves arise from the base of the stem, 
Leaves. or from a root-stock, they are described as radical ; 
if from the whole length of the aerial stem, as cauline. Foliage 
leaves, borne by the aerial stem, are arranged according to two 
main types: spiral and whorled. When leaves come off singly 
at the nodes, the arrangement is spiral; when two or more 
‘leaves come off at a node, the arrangement is whorled. The best 
way of forming an accurate idea of the spiral arrangement is 
to examine a succession of plants ; for example, rose, almond, 
golden rod, dead-nettle. In rose it will be found that the 
sixth leaf is the one which comes exactly above the first; the 
seventh above the second; and so on. If now a piece of 
twine is fixed to the first leaf, and passed over the bases of 
the intervening leaves until the sixth is reached, the twine 
will have passed round the stem twice. The leaves being 
exactly at an equal distance from each other, it follows that 
- twice the circumference of the stem divided by the number of 
leaves will give the distance of one leaf from the other. In 
this case each leaf is separated from the succeeding one by 
2 of the stem. ‘This is known technically as the divergence of 
leaves. In almond, the divergence is 2; in golden rod, 53, ; 
in dead-nettle, $. The denominator denotes the number of 
leaves between those which lie exactly above each other; the 
numerator, the number of times the twine must be passed 
round the stem before the leaf, which is exactly above the 
one to which it is attached, will be reached. : 
Leaves which are spirally arranged are described as alter- 
nate ; those arranged in whorls are described as opposite, if 
