X] 



DISINTEGRATION OF THE STELE 



Fig. i8o. Series of solenostelic and dictyostelic stems of Ferns, all drawn to same scale. 

 ( X 2.) I, Metaxya; 2, Dipteris conjugata; 3, Jllaionia pectinata; 4, Plagio^yria 

 pycnophylla; 5, Thyrsopteris elegans; 6, Saccoloma elegans; •] , Platycerium alcicorne; 

 8, Platycerimn aethiopiatm. These drawings show that the disintegration of the 

 stele does not depend on absolute size alone. 



conditions on the one hand of endodermal contro 

 absence. The final effect of the 

 stelar changes in the growing 

 stocks of advanced Leptospo- 

 rangiate Ferns is to break up the 

 vascular tracts, as seen in trans- 

 verse section, into relatively 

 small, strictly circumscribed, 

 circular or oval masses, each 

 with a relatively large propor- 

 tion of surface to bulk (Fig. 1 80). 

 The physiological difficulty fol- 

 lowing on increase of size is 

 thus fully met in the stems of 

 Leptosporangiate Ferns. As 

 explained in Chapter ix, a 

 similar disintegration of the 



and on the other of its 



Fig. 181. Transverse sections of petioles, all drawn to the 

 same scale. ( x 2.) i, Diptei'is conjugata; 2, Dipteris 

 Lobbiana ; 3, Metaxya ; 4, Phlebodium aurciun ; 5, 

 Thyrsopteris; 6, Alsophila anstralis. These show that 

 while greater size leads to vascular disintegration, there 

 is no definite proportion. 



leaf-trace also follows on increase in size of their petioles (Fig. 181). 



