124 THE STEM. 



places, and here as many broad plates of cellular tissue, belonging 

 properly to the bark, are interposed, passing at right angles to 

 each other from the circumference towards the centre, so that the 

 transverse section of the wood nearly resembles a Maltese cross. 

 But these are all exceptional cases, which scarcely require notice 

 in a general view. 



209. The wood of the Pine, Yew, Cypress, and the whole tribe 

 of what are called Coniferce, or cone-bearing trees, is character- 

 ized by its uniformity of structure, being formed of a peculiar 

 woody tissue with little or no intermixture of true ducts, and by 

 having the walls of these woody tubes marked with large circular 

 discs, as in Fig. 23 (45, 54). 



210. Sap-W00(l and Heart-WOOd, In the germinating plantlet and 

 in the developing bud, the sap ascends through the whole tissue of 

 whatever sort ; at first through the parenchyma, for there is then 

 no other tissue ; and the transmission is continued through it, espe- 

 cially through its central portion, or the pith, in the growing apex 

 of the stem throughout. But in the older parts below, the pith is 

 soon drained of sap by the demand above, and becomes filled with 

 air in its place : thenceforth it bears no part in the plant's nourish- 

 ment. As soon as wood -cells and ducts are formed, they take an 

 active part in the conveyance of sap ; for which their tubular and 

 capillary character is especially adapted. But the ducts in older 

 parts, except when gorged with sap, contain air ^lone ; and the 

 sap now continues to rise only or chiefly through the stem, year 

 after year, to the places where growth is going on, through the 

 proper woody tissue of the wood. In this transmission, the new 

 and fresh tissues are the most active. The walls of the cells that 

 compose them soon begin to thicken by internal deposition and by 

 incrustation with mineral matters introduced with the sap (39, 40, 

 53) ; and by the formation of new annual layers outside of them, 

 their predecessors are each year removed a step farther from the 

 region of growth ; or rather the growing stratum, which connects 

 the fresh rootlets, that imbibe, with the foliage, that elaborates, the 

 sap, is each year removed farther from them. The latter, there- 

 fore, after a few years, cease to convey sap, as they have long 

 before ceased to take part in any vital operations. This older, 

 more solidified, and harder wood, which occupies the centre of the 

 trunk and is the part principally valuable for timber, &c., is called 

 HEART-WOOD, or DURAMEN : while the newer layers of softer, 



