1909] DORETY—MICROCYCAS CALOCOMA 143 



strands which are brought to the center of the meriphyte and form the 

 flanks of the H. When two of these strands are fusing, the centripetal 

 elements of both xylem groups are gradually eliminated and the 

 protoxylem groups are therefore brought together and finally united. 

 This common group of protoxylem, then, is surrounded by the united 

 metaxvlem, and outside of this the phloem of both bundles may 

 almost encircle the xylem, completing the delusion of a concentric 

 bundle (figs. 15, 16). 



Mucilage ducts occur in stem and leaves. They sometimes 

 extend through the petiole as far as the bases of the pinnae. 



The characteristic cycad ramentum is prominent, especially upon 

 the unfolded leaves. The hairs are one-celled. Fig. 18 shows the 

 t'P of a young pinna bearing these epidermal outgrowths. 



Tannin cells occur in the periphery of the petiole, and may extend 

 tt-ell into the pinnae. They are in close relation with the mechanical 

 tesue, as represented in fig. 17. 



DISCUSSION 



The suppression of one of the cotyledons of Ceratozamia illustrates 

 ne process by which the monocotyledonous condition may be reached; 



the f u 



another method, one i 



Micro 



ory of 



Uj q ' AXX "aniiuil) Willi L11C WC11-K11UV\11 UlCV/xjr ui 



argant. Experimentation with some of the monocotyledonous 



5 



form 



in* to ^ fUSi ° n ° f tHe cot y ledons of Microcycas has a further mean- 

 T he fre^ ? ng&ged in the stud y of a series of juvenile gymnosperms. 



to me to 



many 



represent an ancestral condition of polycotyledony. Many 



'p initios um 



some cases, three 



•3 ; ir lob r - -^ - - — - 



w 'thfou e Cotyedon - A young embryo of Dioon spinulosum 

 in four w r e u 0tyled0nS ' and 0ne of Plnus edulis with twelve cot y ledons 

 the P'ne ^cotT^^ gr ° UpS haVC a remarkable similarity. Of course, 

 end °sperin f° m soon esca P e from the small, comparatively dry 



and thin se ed coats, and develop exteriorly to their full 



