IN) 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. ‘his 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 Mavonta: though in others, such 
Transverse section of axis of seedling of Lygodium japonicum, below the first leaf. A=one 
of the xylem-parenchyma cells. 390. (After Boodle.) 
as Prerts aguilina, 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 Lygvdium japonicum (Fig. 111). 
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 L. Se/ago, or L. clavatum, 
