230 



MORPHOLOGY 



Fig. 520. — Tumboa: staminate " flower " 

 (with bracts removed), showing the six tri- 

 sporangiate stamens united below, and the 

 sterile ovule with long and twisted micropylar 

 tube. — Adapted from Hooker. 



nosperm groups, but in addition to these gymnosperm tracheids there 

 arc 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 

 consists of an axis bearing at its 

 tip two or more sporangia (figs. 

 513, 524), and invested below by 

 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 (monadelphous) 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 Fig. 521. — Tumboa: 

 strobilus of the Gnetales, it is evident that the ovuIate " fl °wer," showing 



, , the enveloping and winged 



nucrosporangia 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 micropylar tube), and the 



r . _, .. . . . nucellus containing the em- 



in certain of the Coniferales there are compound bry0 sac _ Adapted from 

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



