158 THE VASCULAR SYSTEM OF THE AXIS [ch. 



rangiate Ferns is further characterised by the completeness of the endodermal 

 sheaths, which shut off the vascular tissue of the stem from the surrounding 

 parenchyma ; that is, the non-ventilated conducting tracts are strictly de- 

 limited from the ventilated tissues that embed them. This feature is not 

 only seen in the juvenile stem, as is the case in the Higher Plants, but it is 

 maintained throughout life, and equally in the largest axes. In this the 

 Leptosporangiate Ferns differ from the larger primitive Ferns, and indeed 

 they stand alone among the larger groups of Vascular Plants, as they do also 

 in the complexity of their stelar subdivisions. It will be seen in Chapter X 

 that physiological reasons can be assigned for these very distinctive peculiari- 

 ties of their structure. 



Fig. 152. Transverse section of a large bifurcated trunk of Cyathea niedullm-is, 

 showing the relatively small size of the tw^in stems, and the large bulk of the 

 adventitious roots that emVjed them. The two pith-cavities can be followed down 

 through the massive section, and are found to be continuous with that of a single 

 trunk below ; this is in itself evidence of dichotomy. Much reduced. 



It will be obvious that any stem constructed on the lines described in 

 this Chapter will be mechanically unstable. Since it increases in size from 

 the sporeling to the adult, and there is no secondary thickening, the form of 

 the stem is obconical, and in upright plants the cone is balanced on its apex 

 at the level of the soil. The mechanical, and also the physiological problem 

 will be the same as that of the Palm-type among the Monocotyledons; such 



