96 THE LEAF. 
but sometimes, as in the Cornus, it is colored like petals. Situ- 
ated at the base of a compound umbel (305, a) it is called a 
general wnvolucre, at the base of a partial umbel a partial invo- 
lucre, or involucel, both of which are seen in the Umbellifere. 
255. In the Composite the involucre consists of imbricated bracts, often in 
several whorls surrounding the base of the heads (compound flowers), as the 
calyx surrounds a simple flower. 
256. In the grasses, the bracts subsist under the common name of husk or 
chaff, to which is attached the awn or beard. 'The bracts situated at the base of a 
spikelet of flowers, are called the glume, corresponding to the involucre. Those 
situated at the base of each separate flower are palee, answering to the calyx, or 
corolla. The pieces, of which each glume or palea is composed anid two) 
are called valves. 
§10.. DURATION. 
257. Leaves, although so universal an accompaniment of vegetation, are only 
temporary appendages. They rapidly attain their growth, and in a great ma- 
jority of cases flourish bat a single season, at the end of which they perish, 
althongh the plant on which they grew may continue to flourish for ages, To 
mark their duration more accurately, leaves are said to be 
1. Fugacious, when they fall off early, before the end of summer. 
2. Deciduous, when they endure for a single season and fall in autumn. 
3. Persistent, or evergreen, when they remain through all seasons, retaming their 
color until the new leaves of the following spring appear, so that the plant is 
always verdant. In accordance with the last two distinctions, plants are said to 
be DECIDUOUS, or EVERGREEN. 
258. The fall of the leaf in temperate climates, occurs near the end of autumn, 
and marks an important era in the year. ‘The first symptoms of decay are seen 
in the changes of color from green to various shades of gold and crimson. These 
gorgeous hues, gradually fading, at length give place to a pale russet, the com- 
mon color of the faded leaf. 
259. Defoliation, or the separation of the leaf from the stem, is due to several 
causes. During the latter part of the summer, the vessels become clogged by the 
deposition of earthy and solid matter contained in the sap, until they can no lon- 
ger admit the free circulation of the fluids through them. The whole structure 
consequently loses its vitality, dries up, and withers, and is finally cast off at the 
point of articulation, as a dead part is from the living body of an animal. 
§11. PHYSIOLOGICAL STRUCTURE, 
-260. Since the frame-work of the leaf is merely a divergent 
portion of the medullary sheath (200), it must consist essentially 
of the same tissues, namely, spiral vessels accompanied by 
woody fibre, that is, fibro-vascular tissue. § 
