346 BOTANY 



develops, finally, into an embryo, but this does not occur until after the seed 

 germinates. 



Histology 



Except for the presence of true vessels in the secondary wood, Ephedra is 

 much like the typical Coniferse. In the climbing species of Gnetum, there is 

 formed a second cambium ring, outside the original one, somewhat as in Cycas. 



Welwitschia 



Welwitschia mirabilis is an extraordinary plant, with a long tap- 

 root terminating above the ground in a short, thick stem, which 

 bears two enormous, persistent, strap-shaped leaves. The seedling 

 has two cotyledons, which are followed by the single pair of strap- 

 shaped leaves, persisting as long as the plant lives. In the axils of 

 these leaves are borne numerous branching inflorescences. Each 

 branch terminates in a cone, composed of closely set bracts, arranged 

 in four series. The individual flowers are borne in the axils of these 

 bracts. The male flowers have six stamens and a rudimentary ovule ; 

 the female flower has a single ovule, much like that of Gnetum, and 

 no trace of stamens. 



Fossil Gymnosperms 



Many of the existing types of Gymnosperms also occur fossil, 

 being the oldest Seed-plants. The oldest are the Ginkgoales and 

 Cycadales, which are met with first in the later Palaeozoic forma- 

 tions. There are also several types which have become entirely 

 extinct. Most important of these are the Cordaiteae, which are 

 much the oldest seed-bearing plants, occurring abundantly in the 

 Devonian and Carboniferous strata, where they have been preserved 

 with marvellous perfection, even the structure of the ovules and 

 pollen-spores being recognizable. They were slender trees with 

 long leaves, which were first supposed to belong to Monocotyledons. 

 The large pollen-spores sometimes show traces of the male gameto- 

 phyte, which was evidently better developed than in any existing 

 Spermatophytes, and probably gave rise to numerous sperm atozoids. 

 Fertilization was apparently effected much as in the Cycads. 



Cycado-filices. Another remarkable group of extinct plants are 

 the Cycado-filices, including a large number of Palaeozoic forms, 

 intermediate in character between the Ferns and the true Cycads. 

 Many of these were described as Ferns, and probably approach these 

 more nearly than they do the Cycads e.g. Lyginodendron. Others 

 e.g. Cycadoxylon, Cycadospadix were probably more like Cycads 

 than Ferns. 



Cycadales. The Cycadales were especially well developed during 

 the Mesozoic age, when numerous types, closely -related to the living 



