STELAR THEORIES 



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foliar bundle persists, and becomes more obvious, the edges of 

 the xylem being separated by parenchyma. This ground tissue 

 is delimited from the vascular tissue by the endodermis, which 

 is developed around the horns of the vascular tissues. At a still 

 later stage a further elaboration obtains ; the fundamental tissue 

 encroaches, as it were, more and more towards the centre in such 

 a way that ultimately the internal phloem forms a band lining 

 the concavity of the xylem and enclosing the central mass of 

 ground tissue, from which it is in turn separated by an inner 

 endodermis (figs. 2 and 3). 



During these progressive changes the vascular system has 

 widened out ; and further, as the leaf-traces become more 

 numerous, the leaf-gaps overlap ; hence, in any transverse section 

 of a well-developed plant, the vascular system will be found to 





PA/oem. 



Xy/e/rr. 



A 



E> 



Leaf-Trace, 



'0: 



%4 



consist of a number of separate strands of xylem, each surrounded 

 by;a band of phloem and endodermis (fig. 4, c). 



In other words, we have what — following Van Tieghem's 

 hypothesis — would be termed the polystelic condition. 



Doodia aspera may serve as an example for the development 

 of the majority of the so-called polystelic ferns, although, it 

 must be remarked, the type shows modifications in different 

 plants. Thus, in some ferns, e.g. some species of Schizcea, it is 

 found that the solid axial rod of xylem, which obtains in young 

 plants, develops in its centre not phloem but parenchyma. In 

 other cases, the appearance of the internal phloem may be delayed 

 for some time after the first appearance of the central parenchyma, 

 as in some species of Angiopteris, Aspkuium, and Nothochlcena. 

 Then, as regards the internal endodermis, the time when this 

 tissue first makes its appearance also varies in different plants. 



