230 



MORPHOLOGY 



nosperm groups, but in addition to these gymnosperm tracheids there 



are also true vessels of the angiosperm type. 



Staminate strobili. The stami- 

 nate strobili are made up of pairs 

 of decussate bracts, which are im- 

 bricate in Ephedra (fig. 513) and 

 Tumboa '(fig- 519), and connate in 

 Gnetum (fig. 523). In the axils 

 of these bracts are the so-called 

 staminate flowers. In Ephedra 

 and Gnetum a staminate flower 

 FIG. 5 20. Tumboa: staminate "flower" consists of an axis bearing at its 



(with bracts removed), showing the six tri- tip two or more sporangia (figs, 

 sterile ovule with long and twisted micropylar ^ o> J */*. 



tu be. Adapted from HOOKER. two or four bracts, which are free 



or coalescent in a tube. In Tumboa 



the structure is very different and quite remarkable. Within the in- 

 vesting bracts there is a whorl of six united (monad el phous) stamens, 

 each of whose free tips bears three sporangia; 

 and within the cycle of stamens there is a cen- 

 tral (terminal) sterile ovule, whose remarkably 

 long micropylar tube is spirally coiled and 

 broadly flaring at the tip (fig. 520). This re- 

 markable structure indicates that the ancestors 

 of Tumboa had flowers that contained function- 

 ing stamens and ovules, and that in the case of 

 Tumboa staminate and ovulate flowers arose by 

 the disappearance of ovules'in certain flowers, 

 and of stamens in others. No such close 

 association of stamens and ovules is known 

 among gymnosperms, except in Bennettitales, 

 where they occur in the same strobilus. 



In attempting to interpret the staminate 

 strobilus of the Gnetales, it is evident that the ovulate " flower >" showing 



, . the enveloping and winged 



microsporangia are borne upon secondary axes bracts, the two integuments 

 (which are the so-called flowers), and therefore (the inner forming the long 

 the strobilus is compound. In Cordaitales and 

 in certain of the Coniferales there are compound 



FIG. 521. Tumboa: 



micropylar tube), and the 

 nucellus containing the em- 

 bryo sac. Adapted from 



ovulate strobili, but only in Gnetales do com- STRASBURGER. 



