VIl] 



DISINTEGRATION OF THE STELE 



135 



varying- number of xylem-strands. There is external phloem, and probably 

 an internal phloem also existed. If it did, and the line of delimitation be 

 really endodermis, then O. Carnieri is truly dictyostelic. Physiologically the 

 effect is that the ventilated cortex is directly related to the greatly dilated 

 pith, while the vascular tissue is completely shut in by endodermis. The 

 final result in the adult will then be a disintegration of the stele comparable 

 to that seen in the Leptosporangiate Ferns ; but it was achieved by an 

 evolutionary progression quite distinct from theirs. 



The disintegration of the stele in the adult stem and the omission of an 

 effective endodermis, which are foreshadowed in the larger Osmundaceae, 

 are both carried out still more completely in the living genus Ophioglossum, 



4 







m 



mUmm:^ 



m 



^ 



m 



5^ 



•^^ 



Fig. 127. Osniundites Carnieri, '6c\m&\.ex. Arrangement of meristeles. 

 The endodermis is shown by dotted lines. (After Kidston and 

 Gwynne-Vaughan.) 



and in the Marattiaceae. In the young plant of Ophioglossum an outer endo- 

 dermis has been found delimiting the stele, and in some instances even an 

 inner endodermis has been described, as in Botrychiuin (Poirault(i22), Bower, 

 Ann. of Bot. 191 1, p. 540). But in the adult no endodermis is present in 

 stem or leaf (Campbell, Eusp. Ferns, p. 92). The stele of the axis expands 

 into a dictyostele with wide leaf-gaps, which is embedded in sappy paren- 

 chyma (Fig. 128). An extreme condition is seen in the tuberous stocks of 

 O. palniatuni, which may be as much as 2 cm. in diameter, with the sheathless 

 meristeles and petiolar strands scattered through the sappy parenchyma. 



A similar disintegration carried even to a higher degree is seen in the 

 Marattiaceae. Here also an endodermis is present in the young sporeling 

 surrounding the integral stele (Brebner; also Farmer and W\\\, Ann. of Bot. 



