SECT. I 



MORPHOLOGY 



145 



Conifers the wood parenchyma consists of simple rows of cells, which afterwards 

 become filled with resin. 



Besides tracheides {t) and wood parenchyma {hp), other elements 

 take part in the composition of the secondary wood of a Dicotyledon ; 

 these are the vessels (tracheae, g), and the wood fibres (A) (Fig. 

 154 ^, B). The cells of the wood parenchyma ai'e short and have 

 abundant contents, the wood fibres are thick-walled, long cells with 

 pointed ends. The elements with wider l umen s, especially the 



em 



Fic:. 152.— Radial section of a Pine stem, at the junction of the wood and bast, s, Antnmn 

 tracheides ; t bordered pits ; c, cambium ; v, sieve-tubes ; vt, sieve-pits ; tm, tracheidal 

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

 the bast; em, medullary ray cells, with albuminous content, (x 240.) 



vessels, are abundant in the spring wood, in which water conduction 

 is important. The autumn wood, on the other hand, consists of 

 narrow elements, among which the wood fibres, which contribute to 

 the rigidity of the plant, are numerous (p. 79). On account of these 

 differences between spring and autumn wood the annual rings are 

 well marked (Fig. 157), 



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. 154 ef), and the wood fibres. 



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



