SUBKINGDOM SPERMATOPHYTA 



333 



vascular bundles of the stem, very much as in Equisetuin. The branching of 

 the stein is monopodial, the buds arising in the axils of the young leaves. 



fb 



FIG. 295. Sequoia sempervirens. Section of shoot apex (X 25) ; I, I, leaves ; fb, vas- 

 cular bundles. , apex of shoot (X 150); d, dermatogen; pb, periblem; pi, 

 plerome. 



The older stem (Fig. 296) in all Coniferse shows a central pith surrounded by 

 a ring of vascular bundles composed of the united leaf-traces. The bundles are 

 collateral, and the woody portion, or xylem, extraordinarily developed. The 



J 



m 



< 



FIG. 296. A, Pinus Virginiana, cross-section of two-year-old branch. P, pith; 

 a;, wood, showing two annual rings; cam, cambium ; ph, phloem; r, resin-ducts 

 in the cortex. B, P. insignis, cross-section of the inner part of the wood (x 250). 

 P, pith ; t l , primary tracheae ; < 2 , secondary tracheids ; r, resin-ducts ; m, medul- 

 lary ray. 



first-formed woody elements are small spiral tracheids in contact with the pith, 

 and the development of the wood is centrifugal. The secondary xylem is made 



