96 -HE LEAF. 



hut 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 imil^el a ^ja?-^z«Z invo- 

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



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

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

 calyx suiTOunds a simple flower. 



256. In the grasses, the bi-acts 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 flo\vers, are called the glume, coiTesponding to the involucre. Those 

 situated at the base of each separate flower are palecB, answering to the calyx, ot 

 corolla. The pieces, of which each glume or palca is composed (generally two) 

 are called valves. 



§ 10. DURATION. 



257. Leaves, althougli so universal an accompaniment of vegetation, are only 

 temporar , appendages. They rapidly attain their gix)wth, and in a great ma- 

 jority ■ f cases flourish but a single season, at the end of which they perish, 

 although 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 oflf 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 t!ie new leaves of the following spring appear, so that the plant is 

 always verdant. In accordance with the last t^vo 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 

 go'"geous hues, gradually fading, at length give place to a pale iiisset, the com- 

 mon color of the fadcxl leaf. 



259. Ikfoliation, 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 bj^ the 

 deposition of earthy and solid mn.tter 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 np, 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 medullaiy sheath (200), it must consist essentially 

 of the same tissues, namely, spiral vessels accorapanied by 

 woody fibre, that is, fibro-vascular tissue. 



