16 OF WOOD IN GENERAL 



developed on the inner side of the procambium strand and 

 subsequently on the inner side of the cambium sheath. The 

 development of xylem in a procambium strand begins with the 

 conversion of one or a few cells, or vertical rows of cells, of 

 the inner part of the strand into spirally, or occasionally 

 annularly, thickened tracUids or tmchem, known as the protoxylem 

 or first-formed wood. This conversion consists in the loss of 

 their protoplasmic contents, the Hgnification of their walls, the 

 deposit of a spiral thickening band internally, or of a series of 

 rings, and, in the case of trachea, the absorption of the trans- 

 verse walls of the vertical rows of cells. Whilst tracheids are 

 elongated cells, losing their contents, generally becoming Hgnified 

 and having thickened walls, so as to be adapted for the conveyance 

 of air or water, trachece or true vessels differ from them only in 

 being formed by the fusion of vertical rows of cells. In a 

 transverse section the protoxylem is recognizable by the relatively 

 small diameter of its tracheae or tracheids; and, where there 

 is a distinct pith, they may be seen projecting into the outer 

 part of the pith in a discontinuous ring known as the medullary 

 sheath. In longitudinal section the loose rings or spirals of their 

 thickening are usually conspicuous, since, being the first vascular 

 elements to form, they are considerably stretched by the growth 

 in length of the adjoining fundamental tissue. The spiral or 

 annular thickening permits, by an uncoiling in the former or 

 a separation of the rings in the latter, a considerable amount 

 of such stretching (Eig. 10). 



The differences between the wood of coniferous trees and that 

 of broad-leaved trees show themselves in the protoxylem and 

 the rest of the primary wood, though they are even more im- 

 portant in the secondary xylem, i,e, that formed after the cam- 

 bium-ring is complete. We will, therefore, now deal with them 

 separately, taking the simpler type, that of the conifers, first 



(Fig. 11). 



The xylem of conifers, both primary and secondary, consists 

 mainly of tracheids ; but tracheae, or true vessels, occur in the 

 protoxylem. In addition to the protoxylem the primary wood, 

 i.e, that which is formed direct from the inner cells of the pro- 

 cambium strand, contains other wider tracheids with bordered pits 

 between the turns of their spiral thickening. 



A cross section of a Piae or Spruce shows distinct annual rings 

 each made up of an inner, softer, Hght-coloured portion, the 

 s'priTig wood, and an outer, firmer, darker-coloured portion, the 

 summer wood. The outer zone of the wood, that next to the bark, 

 comprising from 30 to 50 of the most recently formed of these- 



