i88 



Practical Plant Biology. 



and passes gradually into that formed in the late summer which 

 has thick walls and narrow tubes. The latter is denser, there 

 being more wood substance in it. The recommencement of 

 growth in the spring abruptly starts with the large tubes, which 

 form porous wood. 



The succession of layers of wood formed in the successive 

 seasons is clearly marked out by this abrupt change from late 

 summer to early spring wood. So when a piece of pine wood is 

 cut across you will see a succession of light and dark concentric 

 zones. The light zones are the spring wood and pass outwards 

 gradually into the dark zones which are the summer wood. Out- 

 side each dark zone there is an abrupt change and on its outside 

 comes the spring wood of the next year. Evidently this zonation 

 enables you to tell the age of a branch or stem when cut across, 



each year being represented 

 L by a ring in the cross section. 

 These year-rings are not of 

 uniform thickness and their 

 comparalive sizes constitute 

 a record of the history of the 

 tree, and shows how the suc- 

 ceeding years were favour- 

 able to growth or the reverse. 

 In longitudinal sections of 

 the wood these zones appear 

 as parallel bands, each zone 

 representing a hollow tube 

 of wood. If the longitudinal 

 section is radial, i.e. running 

 through the axis of the stem, 

 the spacing of the parallel bands depends directly on the thick- 

 ness of the year-rings ; if, however, the longitudinal section 

 does riot pass directly through the axis it is tangential, i.e. tan- 

 gential to some cylindrical surface concentric with the pith, and 

 the spacing of the parallel bands is normally wider near the middle 

 line of the section. In oblique tangential sections evidently the 

 year-rings will appear as ellipses, and in this way they will produce 

 the beautiful figure seen in panels of wood cut tangentially and 

 slightly obliquely in the stem. 



The structure of the roots differs somewhat from that of the 

 stem. In them a single axial procambial core of elongated cells 

 gives rise to the conducting tracts. The protoxylem is differenti- 

 ated at a few positions on the surface of this cylindrical core, and 



HMD 



FIG. 56. Pinus silvestris, cut surface of 

 branch nine years old. a, bark ; b, 

 spring wood ; c, summer wood ; d, 

 pith. 



