488 PART III. THE CLASSIFICATION OF PLANTS. 



go: in Torreya and Taxus (Fig. 308) there are no macrosporopli.ylls, the macro- 

 sporangia being borne singly at the end of short lateral shoots, and the seed 

 has a fleshy arillus. Tixus has no resin-ducts in its tissues : its microsporo- 

 phyll is peltate, bearing 5-9 microsporaugia on its under surface. 



Order 3. Gnetaceae. This order includes but three genera, Ephedra, 

 Gnetum, and Welwitschia. Though they differ widely from each other in 

 many respects, they agree in that they have opposite leaves ; flowers which are 

 not cones and which have a rudimentary perianth, but have no macrosporophylls 

 as the macrosporangia are borne on the axis ; an albuminous erect seed ; 

 a dicotyledonous embryo ; and secondary wood which contains true vessels. 

 They are generally dioecious. 



The flowers are arranged in single or compound spicate inflorescences, a-ising 

 either singly (Welwitschia) or in clusters from the axils of pairs of opposite and 

 decussate bracts. The bracts are highly developed and closely packed in Wel- 

 witschia, so that the whole inflorescence has a somewhat cone-like appearance. 



The perianth, in all cases uncoloured, consists of a single pair of coherent 

 perianth-leaves, except in the microsporangiate flowers of Welwitschia, where 

 there are two decussate pairs of free perianth-leaves. 



The microsporangiate flower, in addition to the perianth, consists : in 

 Ephedra, of a short projecting axis bearing 2-8 microsporophylls represented 

 by sessile or shortly- stalked anthers each bearing two microsporangia; in Gnetum, 

 of a short projecting axis directly bearing one or two microsporangia ; in Welwit- 

 schia, of six monadelphous microsporophylls with well-developed filaments each 

 bearing a terminal anther with three microsporangia, surrounding a rudimentary 

 macrosporangium, with a projecting integument, in the centre of the flower. 



. The macrosporangiate flower, in addition to the perianth, consists, in all 

 three genera, of a terminal macrosporangium, borne on the axis ; it is invested 

 either by a sing'e integument, as in Ephedra and Welwitschia, which is pro- 

 duced into a long projecting micropyle ; or by two integuments (Gnetum) the 

 inner of which forms a long projecting micropyle. 



The fruit is formed : in Ephedra, by the succulent development of some of 

 the bracts of the inflorescence which assume a red colour and enclose the fruit- 

 lets formed from the several flowers by the lignificatton of the perianth round 

 the seed ; in Gnetum, by the succulent development of the perianth and the 

 lignification of the outer integumeut, in each separate flower; in Welwitsehia, 

 by the further development of the bracts, which assume a bright scarlet colour, 

 and by the growth of the perianth around the seed so as to form a wing-like 

 expansion, the winged seeds being set free by the falling to pieces of the cone- 

 like inflorescence from below upwards. 



Ephedra is a genus of shrubby plants, with rudimentary leaves, somewhat 

 resembling an Equisetnm. It is especially remarkable on account of its 

 peculiar embryogeny (see p. 471). Habitat, warmer temperate zone. 



Gnetum is a genus of shrubs or trees, for the most part climbers, but some 

 erect-growing (Gnetum Gnemon] : with its broad well- developed foliage-leaves, 

 with pinnate venation, it resembles the Dicotyledons in habit. Habitat, the 

 tropics. 



Welwitschia includes the single species W. mirabilis : it is remarkable for its 

 short thick stem, prolonged below into a tap-root, with a broad flat somewhat 



