DEFINITIONS AND DESCRIPTIONS. 



31 



Fig. 3. 



5. Cells are of three kinds. 

 (1) Those of the bark and pith, 

 of an elhpsoidal form. (2) Elon- 

 gated cells of tlie liber aiul wood. 

 Tliese cells constitute the in- 

 terior of the hark, and are the 

 basis of wood]) fibre. (3) Cells 

 of the medullarrj rays. The 

 medullary rays pass from the 

 pitii to the bark through the 

 wood, as in Fig. 2. c m m m. 

 These cells also have intercel- 

 lular canals. 



6. Spiral vessels exist also in the more i)erfect plants. 

 They are called spiral because they are fibres twisted 

 like a cork-screw, (Fig. 3,) around an empty space. 

 These cells occur only in wood, and are found in bun- 

 dles. Each bundle contains about thirty or forty spi- 

 ral tubes. A new bundle is formed every year, con- 

 stituting the annual layer of icood, or concentric rings, 

 as in Fig. 2. They are supi)osed to be air vessels. In 

 grasses and grains these spiral vessels constitute the 

 part around the interior of the hollow stem. 



7. Pores are oblique openings or slits in the epider- 

 mis or cuticle, so small tJiat a square inch of the e[)ider- ''''^^:i:>^ 

 mis of the kidney bean contains more than 300,000 £^ 

 pores. These pores are found chieliy on the under side of the 

 leaf, and are the organs of transpiration. 



8. Epidermis or cuticle is a very thin 

 membrane which envelopes the soft 

 parts of plants, as the leaves. It is 

 sometimes composed, in part, of silica. 



9. Wood is formed of bundles of 

 spiral tubes, (See Fig. 2,) surrounded 

 by elongated cells. The inner hark, 

 (Fig. 4. 1,) is called the liber, and in 

 the spring a mucilaginotis matter call- 

 ed cambium is interposed between the 

 liber and the wood ; Irom this the 

 first soit wood is formed, called albur- 

 num, which is annually attached to 

 the tree, while a thin layer is formed 

 on the liber or bark, 2, 3, 4, 5. 



