760 A TEXTBOOK OF THEORETICAL BOTANY 



The perforations of the vessels differ from those of Ephedra in consisting 

 of one large aperture only (Fig. 762). This is more like the typical form of 

 vessel in the Angiosperms, but it is produced by the confluence of a number 

 of bordered pits as in Ephedra, and thus is radically different in origin from 

 the vessel pores of the Angiosperms, in spite of a superficial resemblance. 



A further distinction from the structure of Angiosperms is found in the 

 phloem. The sieve tubes and their companion cells originate from separate 

 cells in two different layers of the cambium instead of, as in the higher group, 

 from the subdivision of single cells. 



Fig. 760. — Gnetum gnemon. A branch showing dichasial 

 branching and leaves of the dicotyledonous type. 

 Much reduced. 



In the branching of Gnetum there is commonly more than one axillary 

 shoot from each leaf axil of the primary stem, and these side shoots are 

 differentiated into indeterminate or long shoots and determinate or short 

 shoots, the latter bearing one to eight pairs of decussately set leaves. The 

 leaves on the long shoots may be either normal foliage leaves or, in the 

 climbing type, may be reduced to scales. 



Reproduction. 



Most of the species are normally dioecious, but reduced, functionless, 

 female inflorescences may occur on male plants (cf. Wehoitschia). Both 

 male and female inflorescences are strobili, several centimetres long, arising 

 in the leaf axils. At the base stands a pair of bracts from between which 

 rises the main strobilus, and lateral strobili also grow from the axils of these 

 basal bracts. All of these may branch again, but unbranched strobili are 

 characteristic of some species. 



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