INTRODUCTION 
Position and 
Arrangement 
character is called vernation , from the Latin ver , spring, the season of 
leaf-buds. 
In position, leaves are unhappily termed radical (Latin radix , a root), 
as if they sprang from the root, which they do not, when they are given 
off by an underground stem, as in the Primrose ; whilst 
those given off by aerial stems are called cauline. In 
arrangement, the most fundamental distinction is that 
between scattered leaves, which spring singly from each node of the stem, 
and whorled leaves, which are given off two or more together. In the 
former case many interesting variations of phyllotaxis or leaf-arrangement 
occur, according as the leaves form two, three, five, eight, thirteen, or more 
vertical rows up the stem. Where two leaves form a whorl, they are 
termed opposite ; and in most cases, as in the Labiate Family and most St. 
John’s-worts, the successive pairs of leaves on a stem are at right angles to 
one another, or decussate , as it is called. Similarly, in the rare case in which 
three leaves form a whorl, as in Juniper, each leaf of one whorl stands over 
the space between two leaves of the whorl below. In many cases the 
actual position of the blades of leaves is altered from that indicated by their 
arrangement on the stem by a twist in the petiole, so that the blades form 
a pattern or leaf-mosaic , with little or no overlapping. The rationale of 
leaf-arrangement is to secure the maximum of light and air. 
Among the distinctive characters of the main divisions of Flowering 
Plants few are of more importance than the venation or arrangement of 
the so-called veins, ribs, or nerves of the leaf, those 
conspicuous lines of vessels which convey food-materials 
from the roots, and elaborated sap back to the body of the plant, and 
which remain as a skeleton when the cellular tissue of the leaf perishes. 
One main type is parallel-veined, as in Lilies, Orchids, Grasses, etc., in 
which the principal veins, after diverging at the base of the leaf, run 
approximately parallel and generally converge once more at the apex ; 
while cross-veins are much finer and less conspicuous, and form with the 
longitudinal ones an almost rectangular mesh-work. The other main type, 
known as reticulate or net-veined, may have either one principal vein or 
Venation 
XXIV 
