90 



THE LEAF 



Alternate, that is one after another; or with only a single leaf to 

 each node ; 



Opposite, when there is a pair to each node, the two leaves in this 

 case being always on opposite sides of the stem ; 



Whorled or verticillate, when there are more than two leaves on a 

 node, in which case they divide the circle equally between them, form- 

 ing a verticel or whorl. When there are three leaves in the whorl, 

 the leaves are one-third of the circumference apart; when four, one- 

 quarter ; and so on. So the plan of opposite leaves is merely that of 

 whorled leaves, with the fewest leaves to the whorl ; namely, two. 



161. Phyllotaxy of alternate leaves. Alternate leaves are distrib- 

 uted along the stem in an order which is tolerably uniform for each 

 species. The arrangement in all its modifications is said to be spiral, 

 because, if we draw a line from the insertion (i.e. the point of attach- 

 ment) of one leaf to that of the next, and so on, this line will wind 

 spirally around the stem as it rises, and in the same plant will commonly 

 bear the same number of leaves for each turn 

 round the stem. That is, any two successive 

 leaves will always be separated from each 

 other lay an equal portion of the circumfer- 

 ence of the stem. The distance in height 

 between any two leaves may vary greatly, 

 even on the same shoot, for that depends upon 

 the length of the internodes, or spaces between 

 the leaves; but the distance as measured 

 around the circumference (the angular diver- 

 gence, or angle formed by any two successive 

 leaves) is practically the same. 



162. Two-ranked. The greatest possible 

 divergence is, of course, where the second leaf 

 stands on exactly the opposite side of the stem 

 from the first, the third on the side opposite 

 the second, and therefore over the first, and 

 the fourth over the second. This brings 

 all the leaves into two ranks, one on one side 

 of the stem and one on the other, and is 

 therefore called the two-ranked arrangement. 

 Next is the 



163. Three-ranked arrangement, that of 

 all Sedges, and of White Hellebore. Here 



the second leaf is placed one-third of the way round the stem, the 

 third leaf two-thirds of the way round, the fourth leaf accordingly 

 directly over the first, the fifth over the second, and so on. That is, 

 three leaves occur in each turn round the stem, and they are separated 

 from each other by one-third of the circumference (Fig. 79). 



79. Three-ranked ar- 

 rangement, shown 

 in a piece of the 

 stalk of a Sedge, 

 with the leaves cut 

 off above their 

 bases ; the leaves 

 are numbered in 

 order, from 1 to 6. 



