4-6 



ANALYTICAL CLASS-BOOK OF BOTANY. 



node, as in the Apple and the Oat, fig. 1, Plate XI., 

 and the Evening Primrose, fig. 8 ; 



(2) Opposite, when there are two to each node, as 

 in the Maple, Lilac, and fig. 3 ; and 



(3) Verticillate, when they surround the stem in 

 a whorl, or ring, as in Cleavers and in the Madder, 

 fig. 4. 



2G1. This branch of the science has of late become 

 much expanded ; and the arrangement of leaves has been 

 found obedient to certain mathematical laws, for an ac- 

 count of which the inquirer is referred to " Gray's Bo- 

 tanical Classbook " — a most excellent elementary treatise 

 on the Life, Structure and Habits of Plants, with which 

 no American student should be unacquainted. 



VENATION. 



262. The manner in which the veins are distributed 

 in the leaf is termed Venation. This system is divided 

 into the Mid-vein, the Veins, Veinlets and Veinulets. 



263. The Mid-Vein is the strong process that usually 

 passes through the middle of the blade, from the base to 

 the apex, as in figs. 2 and 5. 



264. Veins are subdivisions of the mid-vein, as in 

 the Cherry, fig. 2, and the Water Chestnut, fig. 5. 



265. Veinlets are radiations from the principal 

 veins, as in fig. 2. 



266. Veinulets are still more remote subdivisions, 

 the ultimate radiations of which are often so fine as to 

 elude the naked eye, as in figs. 12 and 14. 



267. The stronger branches, or primary veins, are 

 frequently called Ribs, and the leaf is said to be three, 

 five, or seven-ribbed, as in figs. 10 and 15 ; while the cen- 

 tral process is called the mid-rib. The word Nerve is 

 also sometimes used to express venation. 



268. There are three principal modes of venation — 

 Pieticulated Venation, Parallel Venation, and Forked 

 Venation. 



269. Reticulated Venation is a characteristic of 

 Exogens, as in the Oak, fig. 1. Such leaves are fre- 

 quently called net-veined leaves, which is, indeed, the 

 shorter and better name. Net-veined leaves are of two 

 varieties — the Radiately-veined, and the Feather-veined 

 leaf. 



270. When the apex of the petiole divides into 

 several nearly equal processes, which radiate from the 

 base to the circumference, the leaf is Radiately veined, 

 as in the Mallow and Castor-oil plant, figs. 15 and 10. 



271. Featheb-veined Leaf. — In this the petiole is 

 obviously continued, and runs from the base nearly or 

 quite to the apex, giving rise to veins, which again throw 

 off their divisions and subdivisions, forming a kind of 

 network, as in figs. 2 and 12. 



272. Parallel Venation. — In this the veins are 

 usually straight, as in the Grasses, fig. 1 1 , and the Palms, 

 fig. 13 , but they are sometimes curved, as in the Orchis, 

 fig. 6. A fine example of this mode of venation is found 

 in the Indian Corn. At fig. 9 is seen a leaf of an Acacia, 

 native of New Holland, which is of a peculiar structure. 

 At the base is a kind of leaf-like petiole, which is parallel- 

 veined ; while the summit is compound and neWeined. 

 Such a leaf is called a Phyllodium. Endogens are dis- 

 tinguished by parallel venation, as in the Lily, Grass 

 and Palm tribes. 



273. Forked Venation. — In this mode — ^which is 

 a characteristic* of Ferns — the veins are thrown off in 

 two opposite pairs, or forked, as in fig. 7. 



CHAPTER XIX. 



leaf-fokms. 



274. Origin of Form. — De Candolle, the great 

 French Botanist, has a theory that the forms of leaves 

 depend on the quantity of parenchyma, and the distribu- 

 tion of their veins ; but for our purpose, the simple facts 

 in the case will be sufiicient. 



275. Familiarity with Leaf- forms is a great help in the 

 discrimination of species, and you will now give your at- 

 tention to the chief of these— merely observing, aswe pass, 

 that the names which distinguish them, being types of 

 mere form, are also applied to all other parts of the plant. 



Whenever a leaf consists of a single piece or part, 

 it is Simple, however much the margin may be divided, 

 as in the Oak, Rose and Lilac. Simple leaves are 



(1) Orbicular, in their nearest approach tO' the cir- 

 cular, as in the Lettuce-Liverwort and Pennywort, figs. 

 1 and 20, Plate XII. No leaf is perfectly round; 



(2) Ovate {egg-shaped), rounded, with the stalk 

 attached to the larger end, as in fig. 2 ; 



(3) Obovate, rounded, with the stem attached 'to 

 the smaller end ; the reverse * of ovate, as in fig. 3 : 



* The particle 06, in Botany, always reverses the meaning of the words to 

 which it is afOsed. 



How with two leaves in each node? In a whorl. Arrangement of the scribe Ecticnlatod venation. What sometimes called? What varieties ? What 

 veins. What called? Parts of the venous system. Define the Mid-vein, great class characterize ? Parallel venation. Varieties. What great class does 

 Veins. Veinlets. Telnnlets. What are the strong primary veina sometimes it indicate? Forked venation. In what plants ! De Oandolle's Theory. Loaf 

 called ? What other word usod ? Three principal modes of Venation. De- | Names— Forms. Define each. 



