S-i THE LEAF. 



de3cribiiig one turn of an elongated spire, so that eacli sixth leaf only is placed 

 exactly above the first. ; 



b. In the f:tnrt!y alternate aiTnno;ement^Ave shall have made one comiilctc turn 

 on arriving at every tliird leaf. But this is rare. jMorc commonly tlic third leaf 

 is a little to the right or left of the perpendicular line on which the iirst is in- 

 serted, so that several turns must be made before we arrive at one which is 

 exactly in that line'. ) 



c. The opposite, or whorled, arrangement Inay be referred to the non-develop- 

 ment of some of the internodes; but a better theory is that which supposes seve- 

 ral coordinate spires arising side by side: two, when the leaves are opposite, and 

 three, or more, when they arc whorled. Tor the leaves of tlie second pair, or 

 whorl, are never placed exactly above those of the first, but above their intei-vening 

 spaces, in accordance with the alternation of the petals with the sepals,j&c. (61, /<)• 



220. Li regard to their position upon the plant, /leaves are 

 radical, when they grow out of the stem at or beneath the sur- 

 face of the ground, so as to appear to grow from the roots ; caic- 

 line, when they grow from the stem, and ramial {ratJius, a 

 branch) when from the branches.^ 



§3. ORGANOGRAPHY. 



221. A leaf may be regarded^.tis an expansion of the two 

 outer integimients of the bark (205) extended into a broad, thin 

 surface by a woody franicAVork, or skeleton, proceeding from the 

 medidlary sheathj(200). This broadly expanded part is caUed 



nhe LAMINA, or BLADE of the leaf, and it is either sessile, that is, 

 connected to the stem by its base, or it is petiolate, connected to 

 the stem by a foot-stalk called the petiole. 1 



222. The petiole, therefore, where it exists, js the unexpanded 

 part of tlie leaf, but like the claws of the petals (102), it is not 

 an essential part, and is often wanting. > Its form|is rarely cyhn- 

 dric, but is usually flattened or channeled on the upper side.^ It 

 i;; said to be 



1. Cb;;?7?;T55e(:/, ^V/hen it is fla^ttened in a vertical direction, so 

 that it is agitated by the slightest breath of air, as in the aspen 

 (Populus). 



2. Winged (margined), when it is flattened or expanded later- 

 ally into a border. Ex. orange. 



3. Amj^lexicaul ( sheathing), Avhen it is dilated at the l)ase into 

 a margin which embraces or surrounds the stem, as in llie Urn- 

 bellifera". 



