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

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



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

 several whorls sun-ounding 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, coiTesponding to the involucre. Those 

 situated at the base of each separate flower are ■palcm, 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, althougli 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 whicli they perish, 

 alUiough 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 oflp 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. 



238. The fall of the leaf in temperate climates, occurs near the end of autumn, 

 and mai-ks 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. The.«e 

 gorgeous hues, gradually fading, at length give place to a pale russet, the com- 

 mon color of the faded leaf. 



259i Defoliation, or the separation of the leaf from the stem, is due to several 

 causes. During the latter part ofthe 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 oif at tlie 

 point of articulation, as a dead part is from the living body of an animal. 



HI. PHYSIOLOGICAL STRUCTURE. 



260. Since the frame-work of the leaf is merely a divergent 

 portion of the medullaiy sheath (200), it must consist essentially 

 of the same tissues, namely, spiral vessels accompanied by 

 woody fibre, that is, fihro-vascular tissue. 



