CYCADALES 



85 



plasm was already known. Intermingled with the tracheids are some- 

 times rows of thin-walled cells, as in Dioon spinulosum (fig. 75). 



The rays in Dioon spimdosiim are usually i cell wide, but often 2 

 cells, and occasionally 3 cells, wide. Longitudinally, they vary from 

 I cell to 20 cells or more (fig. 76). In Cycas media some of the rays 

 are even 6 or 7 cells in width, and are correspondingly long. The 

 cells of the rays contain a large amount of starch but, instead of the 



Fig. 75. — Dioon spinnlosum: longitudinal section of mature wood, showing multi- 

 seriate bordered pits, and also one of the thin-walled cells {t); X390. — After Chamber- 

 lain.'"' 



starch, a cell may include a large crystal of calcium oxalate. The 

 thin-walled cells of the xylem, which also contain starch, are usually 

 in contact with the rays (fig. 76). 



The cambium, with a little of the xylem and phloem, is shown in 

 fig. 77. The bast tracheids have the same width as the other cells of 

 the phloem. While the phloem has not been studied critically in 

 enough forms to warrant a generalization, it may be that such a com- 

 parative study of Cycadofilicales, Bennettitales, and Cycadales 

 would be worth while. 



