THE GYMNOSPERMAE : CYCADALES, ETC. 747 



representing the forked scales. These leaves therefore appear to have two 

 normal and two reduced segments, one above the other. 



Relationships 



Ginkgo occupies such an isolated position that its relationships are 

 uncertain. Mention has been made of the fossil genus Baiera (Permian to 

 Jurassic), which was certainly a close relative. In it the following characters, 

 which we have noted as abnormal in Ginkgo, are the rule ; namely, multilobed 

 leaves, numerous ovules branching from one peduncle and numerous micro- 

 sporangia on each sporophyll. Another genus, Trichopitys (Permian), may 

 also belong here. It has forked leaves with narrow segments, and above, 

 but not in, the leaf axils there are ovuliferous branches with numerous, 

 apparently pinnate, sporophylls, each bearing a terminal ovule. Beyond 

 the suggestion of relationship to these two genera we have little to guide us, 

 but there can be little doubt that the family arose from an early Pteridosperm 

 stock, probably before the Cycads, and has remained distinct ever since 

 Palaeozoic times. 



Gnetales 



The Gnetales are usually included in the Gymnospermae on the ground 

 that in them pollination is still an ovular function and that no style or stigma 

 is present, as it is in all typical Angiosperms. The ovules are, however, 

 enclosed within coverings additional to the true integument, and these are 

 sometimes considered to be the equivalent of ovary walls. 



A common character of the group is the prolongation of the apex of the 

 integument into a long narrow tube with a flattened orifice, by means of 

 which the pollen is collected. The organs producing the microspores resemble 

 the stamens of Angiosperms, consisting of a short axis or stalk bearing a 

 terminal group of more or less united microsporangia. 



Once more ,we have to deal with a group of plants containing a small 

 number of very distinctive and widely scattered forms, which have the 

 appearance of being the remains of a formerly larger and more united order. 

 Fossil remains of the group are, however, remarkably few and do not go 

 back beyond the Tertiary period, so that we have little idea of their ancestry. 

 The living forms are all highly specialized in structure and have departed so 

 far from the primitive gymnospermic stock that they are probably of fairly 

 recent appearance. Moreover they are predominantly plants of dry situations, 

 and plants growing under such conditions are unlikely to be fossilized 



(Fig- 750)- 



They are so advanced in structure that in several respects they approach 



the Angiosperms. No direct relationship can, however, be considered 



possible. Whatever the ancestry of the Angiosperms may have been, it was 



at least not among the Gnetales. They are a parallel line of evolution and, 



so far as we can see, a blind alley, but not only are they morphologically 



striking in themselves, they also show us the summit of gymnospermic 



development from a cordaitalean ancestr\\ 



