LESSON 10.] ARRANGEMENT OF LEAVES. 71 



LESSON X. 



THE ARRANGEMENT OF LEAVES. 



180. UNDER this head we may consider, 1. the arrangement of 

 leaves on the stem, or what is sometimes called PHYLLOTAXY (from 

 two Greek words meaning leaf-order) ; and 2. the* ways in which 

 they are packed together in the bud, or their VERNATION (the word 

 meaning their spring state). 



181. Pliyllotaxy, As already explained (48, 49), leaves are ar- 

 ranged on the stem in two principal ways. They are either 



Alternate (Fig. 131, 143), that is, one after another, only a single 

 leaf arising from each node or joint of the stem ; or 



Opposite (Fig. 147), when there is a pair of leaves on each joint 

 of the stem ; one of the two leaves being in this case always situ- 

 ated exactly on the opposite side of the stem from the other. A 

 third, but uncommon arrangement, may be added ; namely, the 



Whorled, or verticillate (Fig. 148), when there are three or mort> 

 leaves in a circle (whorl or verticil) on one joint of stem. But this 

 is only a variation of the opposite mode ; or rather the latter ar- 

 rangement is the same as the whorled, with the number of the 

 leaves reduced to two in each whorl. 



182. Only one leaf is ever produced from the same point. When 

 two are borne on the same joint, they are always on opposite sides 

 of the stem, that is, are separated by half the circumference ; when 

 in whorls of three, four, five, or any other number, they are equally 

 distributed around the joint of stem, at a distance of one third, one 

 fourth, or one fifth of the circumfer- 

 ence from each other, according to 



their number. So they always have 

 the greatest possible divergence from 

 each other. Two or more leaves be. 

 longing to the same joint of stem 

 never stand side by side, or one 

 above the other, in a cluster. 



183. What are called clustered or fascicled leaves, and which 



FIG. 139, Clustered or fascicled leaves of the Larch 



