14 GYMNOSPERMS 



The secondary wood is in the form of plates, mostly one or two 

 cells thick, with medullary rays usually one to four cells thick sepa- 

 rating them. 



The cells of the primary phloem are small, irregularly arranged, 

 and soon become crushed as in living plants. The cells of the second- 

 ary phloem arc as regularly arranged as those of the secondary wood 

 and, in transverse section, a line of phloem cells is continuous with a 

 line of xylem cells formed from the same cambium. The cells of the 



s.s 



Fig. 8. — Lyginopteris oldhamia: x, centripetal primary wood; .v^ centrifugal primary 

 wood; p.x, protoxylem; p, pith; ss, secretory sac; Xioo. — After Williamson and 

 Scott. 



phloem are alternately larger and smaller, the smaller ones dividing 

 at a right angle to the row of cells so as to make a short transverse 

 row of two or three small cells. These small cells have a tendency 

 to become suberizcd, a condition which reaches an extreme in Ben- 

 nettitales and in the living Cycadales. 



The pith is large, consisting of parenchymatous cells with scat- 

 tered groups of thick-walled cells called "sclerotic nests." The cortex 

 is sharply differentiated into two layers, the inner one consisting of 

 large thin-walled cells, and the outer one, with plates of thick-walled 

 cells alternating with plates of thin-walled cells, a structure giving 

 great strength combined with flexibility. 



