I30 THE VASCULAR SYSTEM OF THE AXIS [CH. 



the outer xylem being composed of long narrow tracheides for conduction, the inner of 

 short wide tracheides effective for storage. The same holds for Thamnopteris Schlech- 

 tendalii also from the Permian, and here the distinction between the two regions is more 

 marked (Fig. 123). 



(iii) In the more recent Osinundites Dunlopiixom. the Jurassic of New Zealand, there 

 is still a continuous ring of outer xylem : but the inner xylem is replaced by pith. The 

 mode of transition from the one state to the other is indicated by the structure seen in 

 OsniMidites Kolbei from the Neocomian (or Wealden) of Cape Colony, in which there is 

 a "mixed pith'' with wide reticulate tracheides scattered through thin-walled parenchyma. 

 The former resembles what is seen in adult stems of living Osmundaceae : the latter 

 corresponds to what has been shown by Gwynne-Vaughan in an abnormal stem of O. 

 regalis (Fig. 120). Another feature of advance seen in O. Kolbei is the interruption of 

 the xylem-ring at the departure of each leaf-trace, a condition regularly found in modern 

 Osmundaceae : this will be discussed later. 



The fossil Botryopterideae, Zygopterideae, and Osmundaceae thus bridge over in rough 

 stratigraphical sequences the transition from the solid to the medullated protostele. The 

 changes are entirely intra-stelar, and though on a larger scale of size, they correspond to 

 the transition already traced among correlative living Ferns. A similar transition may be 

 followed through a "mixed pith" to a parenchymatous medulla in a roughly stratigraphical 

 sequence in the Lepidodendrons, Sigillarias, and other fossil Pteridophytes. In these also 

 the change is intra-stelar, and may lead to interruption of the xylem-ring. The conclusion 

 follows that not only in Ferns but also in other Vascular Plants there has been a pro- 

 gressive medullation of the xylem. It is effected by conversion of the central region into 

 parenchymatous pith. This change goes along with increase in size of the stele, and may 

 be completed within the barrier of an uninterrupted endodermis. 



Structural advances following on Medullation 



Among primitive Fern-steles further structural changes may follow. Sometimes the 

 pith becomes sclerosed. This is characteristic of xerophytic Ferns, such as Schisaea, and 

 Platyzoma. Its importance may perhaps lie in relation to storage of water in a form in 

 which it cannot readily be exhausted through drought. In others the pith-cells may become 

 storage-places for starch, while the cell-walls split at their angles, forming intercellular 

 spaces, which are absent from the smaller vascular tracts. This condition is seen in the 

 Ophioglossaceae and Osmundaceae, in both of which a ventilated storage-pith is found. 

 In this event, so long as the outer endodermis is an unbroken sheath even at the exit of 

 the leaf-traces, it forms not only a barrier which places the transit of plastic materials under 

 the protoplasmic control of the endodermal cells, but it also cuts off the intra-stelar venti- 

 lating system from the cortical, and encloses it as effectively as the ventilating system of 

 submerged plants is enclosed by their epidermis. A careful structural examination of the 

 adult stem of OsmuHda regalis has revealed no connection between the cortical and the 

 stelar systems of ventilation. Physiologically this isolation of the two systems is a weak 

 point: but it is set right by other means in most Ferns, as will be seen later. 



More important from a theoretical point of view than these changes is the appearance 

 of patches of internal endodermis in the pith, which has now been recorded in adult stems 

 of Botrychium and Hehninthostachys, in Schizaea and Platyzojita, and in Todea and 

 Osmunda. Since the attempt has been definitely made to show that these sporadic tracts 

 of endodermis are relics of degeneration of some more complex structure, it is well to 

 realise how relatively primitive are the organisms in which they are found. Also that the 

 group of cells showing the endodermal characters frequently occur quite isolated from kny 



