DICOTYLEDONS. 64 1 



The Venation 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, Calycanthaceae, Nymphaeaceae, and Nelum- 

 biaceae) are all or some of them arranged spirally {acyclic or hemicyclic ^). 



In Cyclic Floweis 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 apelalous. 

 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 an 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 Flora 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, Bluthendiagramme ; Gray, Stmctural Botany.] 



^ Compare pp. 600 and 608. 



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