THE GYMNOSPERMAE : CYCADALES, ETC. 



713 



Algal layer 



Fig. 715. — Cycas revoluta. Transverse section of a 

 coralloid root showing the zone of cells inhabited 

 by Anabaena cycadaceariun. 



even in old stems, consists of parenchymatous cortex, the cells of which 

 are densely filled with starch grains. This ground tissue is traversed by 

 numerous mucilage canals. The rigidity of the trunk depends, therefore, 

 largely on the highly sclerotic armour of leaf bases already mentioned. This 

 is additionally fortified by the formation of corky periderm outside the 

 cortex, so that the armour and periderm together may be 10 cm. thick. This 

 periderm is first formed in the leaf bases themselves, successive phellogens 

 appearing, centripetally, until the leaf base is finally cut ofl^ at the level of 

 the stem surface. Then another phellogen appears in the outer cortex, 



3a r'\^^^ ^s^ 



Fig. 716. — Cycas revoluta. Diagram of the meri- 

 stematic tissues at the stem apex, i, Group of 

 initial cells. 2, Corpus cells. 3a, Tunica layer. 

 3b, Cortex developing from the flanks of the 

 corpus. 4, Medulla developing from the 

 corpus. The arrows show the directions in 

 which differentiation is taking place. {After 

 Adriance Foster.) 



