DICOTVLEDONS. 
641 
The Vetialion of the fohage-leaves (with the exception of the thick leaves of 
succulent plants) is distinguished by the numerous veins which project on the 
under side, and by their curvilinear anastomoses by means of fibro-vascular 
bundles running through the mesophyll itself. The mid-rib, which usually divides 
the leaf into two symmetrical but sometimes into very unsymmetrical halves, gives 
off lateral veins right and left ; one, two, or three strong nerves, similar to the 
mid-rib, often springing in addition from the base of the lamina right and left 
of the median line. The whole system of the projecting veins of a foliage-leaf 
behaves like a monopodial branch-system developed in one plane, the interstices 
being filled up by the green mesophyll in which lie the anastomoses combined 
into a small-meshed network. Within the meshes still finer bundles are usually 
formed which end blindly in the mesophyll. In membranous cataphyllary and 
hypsophyllary leaves and the perianth-leaves of the flowers the projecting veins 
do not usually occur ; the venation is more simple and more like that of Mono- 
cotyledons ^ 
The Flower ^. In the great majority of Dicotyledons the parts of the flower are 
arranged in whorls, or the flowers are cyclic ; only in a comparatively small number 
of families (Ranunculaceae, Magnoliaceae, Calycanthacese, Nymphaeaceae, and Nelum- 
biaceae) are all or some of them arranged spirally {acyclic or hemicyclic 
In Cyclic Flowers the whorls are usually pentamerous, less often tetramerous, 
both numbers occurring in nearly-related plants. Dimerous or trimerous or combi- 
nations of dimerous and tetramerous whorls are much less common than penta- 
merous, and are usually characteristic of smaller groups in the natural system. 
When the floral whorls are tetramerous or pentamerous, they are generally four 
in number, and are developed as Calyx, Corolla, Androecium and Gynaeceum. In 
dimerous or trimerous flowers the number of the whorls is much more variable, and 
then it is not uncommon for each series of organs to be made up of two or three 
whorls ; while in the previous case the multiplication of the whorls is almost entirely 
confined to the androecium. 
The corolla is frequently absent, and the flowers are then said to be apetalous. 
When the calyx and corolla are both present the number of their parts (sepals and 
petals) is almost always the same {Papaver is an exception) ; but this is not the case 
with the number of the whorls. In Cruciferae, for example, the calyx consists of two 
decussate whorls of two sepals each, the corolla of one whorl of four petals. When 
the perianth and androecium are both present (whether the former consist of calyx 
only or of both calyx and corolla), the number of their parts is usually the same, that 
is, the flower is isostemonous, but the stamens are often more, rarely fewer in number 
than the parts of the perianth, and the flower is then anisostemonous. When the 
' [The structure of the leaf compared with that of the stem has been worked out by Casimir De 
Candolle, Archives des Sciences, 1868; the 'Student' for the same year contains ati abridged 
translation of his paper.] 
^ The floral diagrams given here are drawn partly from my own investigations, but chiefly from 
the researches of Payer into the history of development, assisted by Doll's Plora of Baden. The 
figures placed beneath the diagrams are intended to indicate the number and cohesion of the carpels 
as well as the placentation in those plants the diagram of which is otherwise the same. [See also 
Eichler, Bliithendiagramme ; Gray, Structural Botany.] 
^ Compare pp. 600 and 608. 
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