CHAPTER III. THE LEAF. 



21 



leaf embraces or straddles only one margin of the other, Fig. 37 ; 

 and it is said to be triquetrous when, as in the Sedges, Fig. 38, 

 the leaves are conduplicate, and so arranged that the bud is 

 triangular in cross-section. Other modes of the disposal of 

 leaves with reference to each other will be considered under 

 aestivation or prefloration, when we come to the study of the 

 flower. 



Phyllotaxy. This term has reference to the arrangement of 

 leaves on the stem. It is important to observe that they do not 

 occur at random on a stem or branch, but in a definite order, 

 although the order varies in different plants, and sometimes is 

 different on different parts of the same plant. 



Kig. 39. 



Fig. 40. Fig. 41. 



Figs. 42 and 43. Fig. 44. 



Fig. 3g. — Branch of Privet, showing the opposite and decussate leaves. 

 Fig. 40. — Whorled leaves of the Canada Lily. 



Fig. 41. — Branch illustrating the alternate. % or distichous arrangement of leaves. 

 Fig. 42. — Portion of stem of Carex, with leaves partially cut away, showing the % or 

 tristichous arrangement. 



Fig. 43. — Transverse section of leaves of Carex, showing tristichous arrangement. 

 Fig. 44. — Branch showing 2. or pentastichous arrangement of leaves. 



There are two general plans of phyllotaxy ; first, the whorled 

 or verticillate, and second, the alternate or scattered. 



In the whorled plan two or more leaves occur at a node or 

 on the same level, as in Figs. 39 and 40. Where there are but 

 two leaves at a node they are almost invariably situated 180 

 apart, that is, the circumference of the stem is equally divided 

 between them, and the leaves are said to be opposite. 



Opposite leaves are usually decussate, that is, the second 

 pair stand over the intervals between the first pair, as in Fig. 39. 



