IX] THE PHYLLOPODIUM 169 



blade. They are particularly well seen in the Bracken. Stomata are found 

 on their surface, and below it soft, well-ventilated tissue. They are in fact 

 pneinnathode-areas extending along the leaf-stalk and its branches. It is in 

 relation to these that the deep involutions lie, so that there is ready access 

 from the outside to the deeper-lying tissues for purposes of ventilation. 



In addition to this the vascular tract itself is frequently broken up 

 by perforations, which usually originate at points midway between the 

 protoxylem-groups. Here the xylem may first be interrupted, forming 

 xylic gaps. It is only a step further to the interruption of the meristele. The 

 perforations may appear first on the ab- 

 axial curve (Fig. 162), but more commonly 

 at the base of the involutions. The latter 

 is seen in Thyrsopteris (Fig. 161, 3), and in 

 Histioptetds incisa. In both regions the 

 interruption is very completely carried out 

 in the large leaves of the Cyatheaceae, so 

 that the meristele has the appearance of 

 being resolved into its constituent units, 

 according to the anal3'sis of Bertrand and 



Cornaille (Fig. 161,6). In all of these units, . Fig- 162. Transverse section of the petiole 

 ° o\ Saccolonia elegatis [y. \). The vascular 



however, the complete endodermal sheath strands are black, sclerenchyma dotted : 

 is maintained, in sharp contrast to the the clear areasare ventilated parenchyma, 

 ' ^ connectmg with the pneumathodes/,/. 



breaking up of the meristele in the Marat- 



tiaceae and Ophioglossaceae, where the sheath is absent. The effect is that 

 while the protoplasmic control over the conducting strands is maintained, 

 intercommunication between the external and internal parenchyma of the 

 petiole is fully provided for, even in the largest of the Leptosporangiate 

 Ferns. 



Perforations of the meristele naturally explain the divided leaf-trace. 

 It has been seen that they frequently occur close to the base of the petiole. 

 If they are extended downwards through the cortex of the stem to the very 

 insertion of the foliar meristele, the effect is that in relatively advanced 

 types the leaf-trace is divided from the first; it may be into two straps as in 

 Aspleniicni or Trisineria, or it may be into numerous strands as in Dryo- 

 pteris, and most of the Polypodiaceous Ferns. In relatively primitive Ferns, 

 however, such as Cibotimn Barometz and Saccoloma, the leaf-trace is still 

 undivided at its base. But after traversing the cortex and entering the 

 petiole it soon breaks up into a number of separate strands (Fig. 163), thus 

 indicating structurally the intermediate position of these Ferns. 



In contrast to these effects of expansion certain Ferns show a structure 

 in the phyllopodium which points to contraction of the meristele. In place 

 of a wide curve the leaf-trace is compact and almost cylindrical, while its 



