X] 



VARYING FORM OF PROTOSTELE 



183 



curvatures of the hollows are deeper in the larger than in the smaller speci- 

 mens. A still more extreme case of this is seen in Asterochlaena laxa, which 

 may be as much as I5"5 mm. in diameter (Fig. 175, iv). Here the stele is 

 thrown into deep involutions of the surface. It is obvious that this form will 

 give a very greatly increased proportion of surface to bulk. It seems natural 

 to conclude that in such cases the more elaborate form of the stele has made 

 the larger size possible, by overcoming the limiting factor. But notwith- 



Fig. 1 75. Outlines of xylem of steles, all drawn to the same scale ( x 5), 

 to show approximately relative size, i, Botryopteris cylindrica, 

 diameter •65 mm.; ii, Ankyropteris Grayi, diameter 2*0 mm.; iii, 

 Ankyropteris Grayi, diameter 2 -5 mm. ; iv, Asterochlaena laxa, 

 diameter 12-0 mm. The elaborateness of outline increases with the 

 size. 



standing the complicated outline, and the well-known differentiation of the 

 xylem of these fossils, their steles are still of the nature of protosteles. Their 

 non-medullated structure is maintained. 



In other primitive Ferns, as a larger size of the stele is attained in the 

 growing plant, a change of internal structure appears, leading to vtediillatio7t. 

 Since the leaf-traces are inserted peripherally, it is in the outer xylem that 



