i84 SIZE A FACTOR IN STELAR MORPHOLOGY [ch. 



the water-transit will be most active. As the stele enlarges the water in the 

 central region will tend to stagnate, and thin-walled cells will serve for its 

 storage as well as thick-walled tracheides would do. This is probably the 

 rationale of the condition of " mixed pith," and of the formation of a paren- 

 chymatous medulla. Its intrastelar origin has been traced in Chapters Vil 

 and VIII. It has there been shown how the relatively small stem of the living 

 Osmundaceae may have a ventilating system in the storage-pith quite dis- 

 tinct from that of the storage-cortex. Where the size is small that may be 

 a workable arrangement : it is actually seen in the adult stem of Todea 

 Barbara, 3 mm. in diameter, and in Osmunda regalis, 5 mm. In such cases 

 the proportion of surface to bulk of the stele contained within is relatively 

 high (Fig. 176). But the 

 case is different for the large 

 steles of Osimmdites skidega- ^ O 

 tensis (25 mm. in diameter), 

 or of O. Carnieri (35 mm. in 

 diameter) (refer to Fig. 127, jj r\ 

 Chapter Vll). In them the 

 problem raised by their larger 

 size is solved by breaking 

 down the barrier, leading thus "' r~J 

 to the completion of a com- 

 mon ventilating system for ^ 



J . , „, ,. . . Fig. 176. Traces of the actual size of steles of living and 



cortex and pith, i he limitmg fossil Osmundaceae, all to the same scale, i.e. approx. nat. 



factor is met bv interruption ^^^^' '^' ^' ^^ supplied by Dr Kidston. i= Todea barbara 



^ _ _ _ (^ mm.) ; 11= Osmtindaciiinawomea {4 mm.); m= Osmtmda 



of the endodermis. This is regalis {imm.)\\s=Thamnopterisschlechtendalii(\imm.); 



in point of fact a more effec- cr^S^TJ^^t^mt^^"'''''"" ^'^ "'™'^' '''= ^'""""^'''' 

 tive device than mere exten- 

 sion of its area. 



A method of solving the difficulty similar in effect to that of the 

 Osmundaceae is seen in the living Ophioglossaceae. In Botryckium and 

 Helinmthostachys the young plant has a complete endodermal barrier, as 

 in other Ferns, shutting off the vascular system from the cortex. As the 

 plant advances and the stele enlarges a pith is formed which serves for 

 storage. Intercellular spaces appear in it, but the internal ventilating system 

 is at first shut off from the cortical, and it remains so till the plant is well 

 advanced. As the stem enlarges still further free communication is estab- 

 lished by foliar gaps (see Fig. 124, Chapter Vll), which open outwards to the 

 cortex, but they also open inwards to the pith. This has the disadvantage 

 of laying open the conducting tract, and destroying the completeness of the 

 endodermal control. But it solves the difficulty of communication between 

 the outer and inner tissues, which becomes more acute as the stem enlarges. 



