SECT. I 



MORPHOLOGY 



135 



All the elements entering into the formation of the wood of Dicotyledons can be 

 derived from the two classes of tissue already met with in the Gymnosperms, the 

 tracheal tissue and the parenchymatous tissue of the wood. The tracheides and 

 vessels belong to the former class, while under the parenchymatous tissue are 

 included the wood parenchyma, fibrous cells of greater length but with similar 

 contents (Fig. 145 ef), and the wood fibres. 



The tracheal tissue consists of elements which lose their living contents at an 

 early stage, and in their fully developed condition are in reality only dead cell 

 cavities. In this class are included TRACHEIDES having relatively wide lumina and 

 large bordered pits, and ultimately also spirally thickened walls, which serve as water- 



sni 



FIG. 143. Radial section of a Pine stem, at the junction of the wood and bast, s, Late tracheides ; 

 t, bordered pits ; c, cambium ; v, sieve-tubes ; at, sieve-pits ; tin, tracheidal medullary raj- 

 cells ; sm, medullary ray cells in the wood, containing starch ; sm', the same, in the, bast ;' em, 

 medullary ray cells, with albuminous contents, (x 240.) 



carriers (Fig. 145 A, t) ; VASCULAR TRACHEIDES (gt), with similar functions, but with 

 the structure and thickening of vessels ; FIBRE TRACHKIDES (ft), with small lumina 

 and pointed ends, having only small, obliquely elongated bordered pits, and, in ex- 

 treme cases, exercising merely mechanical functions ; and finally TRACHEA (</), 

 formed by cell fusion, and provided with all the different forms of thickenings by 

 which they are distinguished as annular, spiral, reticulate, or pitted vessels. All 

 vessels function as water-carriers. If they have small lumina and resemble 

 tracheides, they may be distinguished as TRACHEIDAL VESSELS (tg) ; if, as is 

 generally the case, they, have bordered pits on their lateral walls, they 

 are usually provided with tertiary thickening layers in the. form of thin, 

 spiral bands (Fig. 149 m). In the parenchymatous tissue of the wood the 

 cells (Fig. 145 B) generally retain their living contents, and never develop the true 

 bordered pits with a torus in the closing membrane, which are so characteristic 



