96 THE LEAF. 



but sometimes, as in the Cormis, it is colored like petals. Situ- 

 ated at the base of a compound umbel (305, a) it is called a 

 general involucre, at the base of a partial umbel a poMial invo- 

 lucre, or involucel, both of which are seen in the UmbellifersB. 



255. In the Compositae the involucre consists of imbricated bracts, often in 

 several whorls suiTOunding the base of the heads (compound flowers), as the 

 calyx suiTOunds 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 heard. 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 palece, answering to the calyx, or 

 corolla. The pieces, of which each glume or palea is composed (generally 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 but a single season, at the end of which they perish, 

 although the plant on wliich 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, retaining 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 gi-een to various shades of gold and crimson. These 

 gorgeous hues, gradually fading, at length give place to a pale msset, 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, fihro-vasaidar tissue. 



