104 



MORPHOLOGY OF GYMNOSPERMS 



Fig. 8o. — Dioon edule: semi-diagrammatic 

 reconstruction of part of vascular system of em- 

 bryo; cot, cotyledon; ib, tubular part of cotyle- 

 dons; cs, cotyledonary strands; li-li, foliar 

 strands of first leaf; li-li, foliar strands of 

 second leaf; vp, vascular plate; o, protoxylem 

 elements continuing downward into the hypo- 

 cotyl. — After Thiessen (62). 



bial stage of the strand, and 

 that its final course is laid 

 down from the beginning. 



The seedlings of Zamia 

 (Z. floridana and Z. integri- 

 folia) have been studied 

 recently by Matte (68) 

 and are found to confirm 

 the results obtained from 

 the preceding genera. The 

 vascular plate of the cotyle- 

 donary node is a protostele, 

 and each cotyledon receives 

 three vascular strands, as 

 described for Ceratozamia. 

 At the base of the cotyle- 

 don the strands are mesarch 

 or even concentric, but in 

 the middle, region they be- 

 come exarch and remain so 

 to the tip, where transfusion 

 tissue occurs and probably 

 has replaced the centripetal 

 xylem. The tap root is of 

 the usual tetrarch type, be- 

 coming reduced to triarch 

 or even diarch toward the 

 tip. 



The study of the seed- 

 lings of Microcycas by Sister 

 Helen Angela (65) con- 

 firms the general situation 

 described for Ceratozamia, 

 Dioon, and Zamia, and 

 shows some interesting 

 variations. Microcycas is 

 peculiar in the extensive 



