SIZE A FACTOR IN STELAR MORPHOLOGY 



[CH. 



completely investing the veins even to their distal ends. Moreover, as their endodermis is of 



the secondary type, the conducting tracts would appear 

 to be very completely shut in by it. It seems, however, 

 highly probable that means of transit through it for fluids 

 and soluble substances exist which have hitherto escaped 

 detection. Whether or not this is the fact, it is at least 

 indicated structurally that the endodermis of the Lepto- 

 sporangiate Ferns is a more complete protection to the 

 conducting tracts, and at the same time a more impervious 

 barrier than in the Eusporangiate. But while the endoder- 

 mis can be seen as a continuous layer in Leptosporangiate 

 Ferns, even below the sori themselves, it remains there of 

 the primary and more pervious type until the sporangia 

 are matured (see Fig. 208, C, p. 215) : it is only then that 

 its development according to the secondary type is com- 

 pleted, and the impervious sheath is thus closed. (Compare 

 Basecke, 160; and Priestley, 183.) 



The chief steps in the advancing complexity 

 of the vascular system in the Leptosporangiate 

 Ferns have been described in Chapter Vlil, and 

 are known as solenostely, polycycly, perforation, 

 and dictyostely. They may be variously com- 

 bined in the individual stem, and all result in 

 increase of surface in proportion to bulk of the 

 stelar tissues. They follow on a very considerable 

 increase in size of the individual stem, and of the 

 stele contained in it (Fig. 178). This is graphically 

 shown in the series of drawings to the same scale 

 of the stem of a plant of Pteris {Litobrochia) 

 podophylla. The increased complexity is believed 

 to be causally related to that increase in size. Me- 

 dullation may precede it, as it does in Gleicheiiia 

 pectinata (Fig. 179). In others no previous me- 

 dullation may be apparent. The reason for this 

 is probably to be found in the importance of 

 establishing early and complete endodermal con- 

 trol over the conducting system, combined with 

 internal aeration. Both ave secured by the estab- 

 lishment of an endodermal barrier extending across 

 the medulla (Fig. 179). The structural difference 

 between the parenchyma within the sheath (" in- 

 trastelar pith ") from that outside it (" extrastelar 

 pith ") is probably a consequence not of tissue- 

 origin, but of development of the cells under the 



» 



p 



Fig. 179. Plan of stelar con- 

 struction of a juvenile plant 

 of Gleichenia pectinata, after 

 Dr J. M. Thompson, showing 

 in median section the way in 

 which the stele enlarges 

 conically upwards, and widens 

 into a solenostele, with leaf- 

 gaps. Rather above the middle 

 of the figure the endodermis is 

 continuous across the pith, 

 thus separating the lower, 

 " intrastelar," from the upper, 

 "extrastelar," pith. 



