232 



MORPHOLOGY 



FIGS. 525, 526. Gnetum latifolium: 525, branches 

 bearing ovulate strobili ; 526, part of ovulate strobilus. 

 After BLUME. 



of swimming sperms. In Ephedra 

 a remarkable funnel-shaped pollen 

 chamber is developed by the break- 

 ing down of the nucellar tissue (fig. 

 528), which extends to the embryo 

 sac and exposes the necks of the 

 archegonia; hence in pollination 

 the pollen grains may come to rest 

 in contact with the archegonium 

 necks. In Gnetum the tip of the 

 nucellus is more or less disorgan- 

 ized, and this is the only trace of 

 a pollen chamber (fig. 530); while 

 in Tumboa there is not even a 

 trace. 



Female gametophyte. The struc- 

 ture of the female gametophyte in 

 Gnetales is of great interest, for 

 there is an evident approach to- 

 wards the angiosperm condition. 



Ovule. The ovule has 

 two integuments, the inner 

 one forming a long tubular 

 micropyle, which is a 

 marked feature of the 

 group (figs. 521, 528, 530). 

 The nucellus has the usual 

 gymnosperm feature of a 

 heavy mass of sterile tissue 

 overlying the megaspore 

 mother cell (and later the 

 embryo sac). In all the 

 preceding groups of gymno- 

 sperms, except Coniferales, 

 this overlying tissue is 

 beaked and contains a 

 pollen chamber, a structure 

 associated with the presence 



FIG. 527. Ovulate strobili of Gnetum, 

 with fleshy seeds maturing. 



