xxx Definitions. 



closely attached to the stem, perfoliate when the bases of two opposite 

 leaves are so united that the stem seems to run through them, as in 

 the common honeysuckle. 



12. The petiole may have appendages on it quite distinct from the 

 stem, as in the orange; these are called wings; but sometimes the 

 lower part of the leaf, not shaped as a petiole, runs down into the 

 stem, and forms a sort of wing on it ; the leaf is then called decurrent. 

 When the lower part of the leaf or the petiole expands into a sheath 

 surrounding the stem, as in many plants of the orders Scitaminece 

 and Commelinacece, it is called sheathing. When the petiole is 

 attached, not to the base of the leaf, but to its centre or some other 

 part of the under-surface, the leaf is called peltatt, as in many species 

 of order Menispermacece. 



13. The continuation of the petiole running generally through 

 the middle of the leaf, and forming its back-bone, so to speak, is 

 the midrib. The veins of leaves are either reticulated, i.e. forming 

 a network running in all directions, or parallel to one another, 

 and generally to the midrib. This is a distinction of the first im- 

 portance in classification. The main veins are called nerves, 

 and it is sometimes essential to describe leaves as 3- nerved, 

 5-nerved, etc. In fleshy leaves the nerves are often scarcely 

 distinguishable. 



14. Arrangement of leaves with regard to the stem. When most of the 

 leaves of a plant are arranged about the root they are called radical, 

 those of the stem being then called cauline ; when the leaves are in 

 pairs all up the stem, one on each side of it, they are opposite ; when 

 they occur singly, taking each side of the stem in turn, they are 

 alternate ; when each pair of opposite leaves is at right angles to the 

 next pair they are decussate; when the leaves are arranged one above 

 the other in two opposite rows, one on each side of the stem or 

 branch, they are called distichous or bifarious. 1 When three or more 

 leaves surround the stem at one point they are called whorled, 

 or verticelled. 



Note. Opposite, alternate, or whorled leaves are respectively 

 characteristic of many orders, so that it is often essential to notice 

 the arrangement. 



15. The division of leaves. Leaves are either simple or compound : 

 simple when all in one piece, even though they be cut into lobes 

 or segments, as those of the vine, and of most of the genus Hibiscus ; 

 compound, when they are composed of two or more pieces, looking 

 each like a separate small leaf, as those of gram or the nim tree : these 

 divisions are called leaflets. Simple leaves, which are oblong and 

 deeply divided into segments on each side of the midrib are pinnatifid : 

 if the terminal lobe of a pinnatifid leaf is much the largest it is 

 called lyrate : those which are roundish and rather deeply divided 

 into segments towards the petiole are palmate, those divided almost 

 down to the petiole digitate; if the lobes are narrow and very 

 irregular the leaf is laciniate. Of compound leaves, those which are 



1 A green branchlet, with leaves thus arranged, may sometimes be 

 mistaken for a pinnate leaf. 



