4,72 



APPENDIX. 



tracheids are square in cross 

 curious pits. Each of these 



* 



_ PHLOEM 

 PARENCHYMA 



SIEVE TUBES 



- .CAMBIUM 



AUTUMN 



section, Fig. 11) a row of 

 "bordered pits" appears in 

 surface view (that is, in a 

 radial longitudinal section 

 a longitudinal section taken 

 through the centre of the 

 stem) as two concentric circles 

 (Fig. 12). 



The bast, which is separ- 

 ated from the wood by the 

 cambium, consists of sieve 

 tubes and parenchyma cells, 

 the sieve tubes being long 

 narrow cells with the sieve 

 plates on the side walls 

 (Fig. 12). The medullary 

 rays, however, are more com- 

 plex in structure than in 

 Dicotyledons, and are made 

 up of three distinct kinds of 

 cells. Both in the wood and 

 the bast, the middle cells of 

 the rays contain starch, and 

 are therefore called the 

 " starch cells " (Fig. 12). 

 The upper and lower cells 

 of the ray are empty and 

 Pig. 11. stem of Pine. pitted (" tracheidal cells ") 



Portion of a transverse section after in the WOOd portion of the 

 secondary growth. ^ but where ^ ^ tra . 



verses the bast these cells 



contain proteid matter ("albuminous cells"). Thus the 

 rays have special cells for storing starch, for storing proteids, 

 and for conveying water in different directions, in addition 

 to their important function of binding together the suc- 

 cessive shells of wood which are formed each year. 



Figs. 11-13 show transverse, and radial and tangential longitudinal 

 sections of the wood. A radial, longitudinal section is one which 

 passes through the middle of the stem ; a tangential longitudinal 

 section, cne taken peripherally. Thus a radial section runs parallel to 

 the medullary rays in the region in which it is taken, while a tan- 



^- - ' WOOD 



RESIN 

 PASSAGE 



- MED. RAY 



BORDERED 

 PIT 



