DIV. I 



MORPHOLOGY 



151 



y belonging to the Magnoliaceae, is a Dicotyledon with wood 

 composed of tracheides and parenchyma only. 



B. Arrangement of the Tissues in the Wood. In the 

 Gymnosperms (Figs. 175-177) the wood of the stems and roots has 

 thus a relatively simple structure. The tracheides are arranged 

 in regular radial rows (Fig. 175 A), in correspondence with their 

 mode of origin. Since they increase in size mainly in the radial 

 direction, and hardly at all in the tangential and longitudinal 



m 



FIG. 175. A, Transverse section of the wood of a Pine at the junction of two'annual rings. /, Spring 

 wood ; s, autumn wood ; t, bordered pit ; a, interposition of a new row of tracheides ; h, resin 

 canals ; m, medullary rays; g, limit of autumn wood, (x 240.) B, Part of a transverse 

 section of the stem of a Pine, s, Late wood ; c, cambium ; v, sieve-tubes ; p, bast parenchyma ; 

 fc, cell of bast parenchyma containing crystal ; cv, sieve-tubes, compressed and functionless ; 

 m, medullary ray. (x 240. After SCHEXCK.) 



directions, they retain the same form as the cambial cells (Fig. 169). 

 They have large, circular, bordered pits frequently only upon their 

 radial walls : the pits are thus seen in surface view in radial sections 

 (Figs. 70 ,71 A). 



In the wood of most Gyrnnosperms there is relatively little parenchyma. In 

 the Pines, Firs, and Larches parenchyma is found only around schizogenous resin- 

 canals which run longitudinally in the wood (Figs. 175 A, h ; 179 A), and are 

 connected by others which run radially in some of the broader medullary rays. 

 For this reason'considerable amounts of resin flow out from the wounded stem of a 

 Pine or Fir. In the other Conifers the wood parenchyma is limited to simple rows 

 of cells, the cavities of which may later become filled with resin. 



