80 MORPHOLOGY OF STEM*}. 



rigorous vernal growth, with much larger cells, thus marking 

 an abrupt transition from one layer to the next. Besides being 

 finer, the later wood-cells of the season are commonly flattened 

 antero-posteriorly, probably by growing under greater pressure. 



145. Each layer of wood, once formed, remains essentially 

 unchanged in position and dimensions. But, in trunks of con- 

 siderable age, the older layers undergo more or less change in 

 color, density, perviousness to moisture, &c. 



146. Sap-wood (ALBURNUM). In the 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, 

 especially through its central portion, or the pith, in the growing 

 apex of the stem throughout. But, in the older parts below, the 

 pith, soon drained of sap, becomes filled with air in its place, 

 and thenceforth it bears no part in the plant's nourishment. As 

 soon as wood-cells and ducts are formed, they take an active 

 part in the conveyance of sap, for which their tubular and ca- 

 pillary character is especially adapted. But, the ducts in older 

 parts, except when gorged with sap, contain air alone ; and in 

 woody trunks the sap continues to rise year after year to the 

 places where growth is going on, mainty through the proper 

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

 are most active ; and these are in direct communication with the 

 new roots on the one hand and with the buds or shoots and leaves 

 of the season on the other. So, by the formation of new annual 

 layers outside of them, the older ones 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, therefore, after a few years, cease to convey 

 sap, as they have long before ceased to take part in any vital 

 operations. The cells of the older layers, also, usually come to 

 have thicker walls and smaller calibre than those of the newer. 

 Thus arises a distinction sometimes obscurely marked, some- 

 times abrupt and conspicuous into sap-wood and heart-wood. 

 The former is the popular name given to the outer and newer 

 layers of softer, more open, and bibulous wood. The early physi- 

 ologists named it alburnum from its white or pale color. Being 

 more or less sappy, or containing soluble organic matter, and 

 readily imbibing moisture, this part of the wood is liable to decay, 

 and it is therefore discarded from timber used for construction. 



147. Heart-wood (or DURAMEN, so called from its greater hard- 

 ness or durability) is the older and mature portion of the wood, 



