XYLEM. 19 



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 (Fig. 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 

 important in the secondary xylem, i.e. that formed after the 

 cambium-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. It contains, that is, no true vessels or 

 trachea?. In addition to the protoxylem the primary wood, 

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

 procambium strand, contains other wider tracheids with bordered 

 pits between the turns of their spiral thickening. 



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

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

 spring 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 

 annual rings and from one to three or more inches across, is 

 of lighter colours and is known as the sap-wood. Many of its 

 cells are still in a sufficiently active state of vitality to store 

 up starch, at least in winter, though growth is confined to the 

 outermost layer of all, the cambium. The inner rings are darker 



