84 THE LEAF. 



describing one turn of an elongated spire, so tliat eacli sixth leaf only is placed 

 exactly above the first. 



b. In the strictly alternate arrangement, we shall have made one complete turn 

 on arriving at every third leaf. But this is rare. More commonly the third leaf 

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

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

 exactly in that line. 



c. The opposite, or whorlcd, airangement may be referred to the non-develop- 

 ment of some of the intcrnodes ; 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 are whorled. For the leaves of the second pair, or 

 whorl, are never placed exactly above those of the first, but above their intervening 

 spaces, in accordance with the alternation of the petals wth the sepals, &c. (61, b). 



220. Ill 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 ; cau- 

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

 branch) when from the branches. 



§3. ORGANOGRAPHY. 



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

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

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

 medullary sheath (200). This broadly expanded part is called 

 the 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 caUed the petiole. 



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

 part of the 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 

 is said to be 



1. Compressed, when it is flattened 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. Amplexicaid (sheathing), when it is dilated at the base into 

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

 bellifera?. 



