156 ACADEMIC BOTANY. 



381, If we make a radiant section of the wood — that is, a vertical 

 section in the same plane with the medullary ray — the ray will he 

 fully displayed with its shining cells (Pig. 226). These form the 

 Silver grain of Maple, Oalj, etc. They are called muriform tissue (L. 

 murus, wall), because they resemble bricks in a wall. If we make a 

 tangential section (L. tangens, touching) — that is, a vertical section 

 which touches or nearly cuts across the rays — we shall see the wood as 

 it is usually split or sawed (Fig. 227), showing the vertical wood-fibres, 

 with glimpses of the rays. 



382. In the young stem all the wood-cells carry sap, and are there- 

 fore called Sapwood, or Whitewood (L. alburnutn), on account of the 

 usual color. But after a few years the inner cells near the centre are 

 thickened and hardened by solid deposits, and become Heartwood (L. 

 duramen, hardening). These deposits are variously colored, giving to 

 each stem its characteristic hue, — red to the mahogany, cedar, and 

 cherry ; green to the laburnum ; brown to the locust and walnut ; 

 black to the rosewood and ebony (Pig. 228). The grain of the ebony 

 is so fine that it is not discernible when the wood is polished. The 

 Ebony family (which includes the Persimmon) furnishes many valu- 

 able woods as well as fruits. 



LESSON XXXI. 



BAEK— LEAF. 



383-389. Bark. 390. Epidermis. 891-393. Leaf. 394. Respira- 

 tion. 395. Cuticle. 



383. The Bark is separated from the wood — and at the 

 same time kept in communication with it — by a thin semi- 

 fluid tissue (Fig. 229, cd), called the Cambium Layer (L. 

 cambio, I exchange). Of this cambium we shall learn 

 more in the next Lesson. There are three kinds of bark 

 (Fig. 229): 



1. lAber, or inner bark (Gr. endophkevm) ; 



2. Green, or middle bark {m£Sophlceum) ; 



3. Cortex (Suber), or outer bark (epiphlcmm). 



384. Liber. — Liber-cells are of two kinds : 



1. Proper liber-oells, which consist of parenchyma (young 



