IX PTERIDOPHYTES 215 



not a necessary or a direct Consequence of the recumbent position of 

 the embryo of Leptosporangiate Ferns. It may also be added that 

 the dorsiventrality, in cases where it exists, may be initiated in different 

 ways. Such evidence points to its being a secondary condition. 



A further indication that the shoot of Leptosporangiate Ferns is primarily 

 radial is to be seen in the internal structure of the axis. It has been 

 found in a considerable number of cases that the vascular tissue is 

 uniformly developed all round. This is naturally the case in upright 

 radial axes : but, apart from the leaf-insertions, it may even be so in 

 stems which are horizontal, such as Matonia : though in others, such 



P" 



FIG. in. 



Transverse section of axis of seedling of Lygodiumjaponicunt^ below the first leaf. / one 

 of the xylem-parenchyma cells. X^go. (After Boodle.) 



as Pteris aquilina, the vascular system is like the stem itself dorsiventral. 

 In young seedlings it has been shown in various cases that the stele 

 is cylindrical, and it is found to be so even in Ferns which are markedly 

 dorsiventral in the mature state, such as Lygodium japonicum (Fig. in). 

 Such examples indicate again a probability that the radial construction 

 of the shoot was primitive in the sporophyte of Ferns. 



But it may be urged by those who dissent from this conclusion that 

 dorsiventrality is clearly seen in the early embryonic stages of some other 

 Pteridophytes, and especially in the case of various species of Lycopodium. 

 But here also it seems probable that the condition is adaptive rather 

 than primitive : for in the first place the embryo in the genus is singularly 

 inconstant in its form : in some species, such as Z. Selago, or Z. davatum, 



