VIIl] 



PERFORATION 



149 



vascular system may be dissected out as a cylindrical network, with large 

 overlapping foliar gaps, from the margins of which the numerous strands 

 of the divided leaf-trace arise (Fig. 140). Clearly 

 several leaf-gaps will appear in any transverse 

 section. The vascular system is represented by a 

 ring of " meristeles," each delimited by unbroken 

 endodermis. From the point of view of ventilation 

 the numerous, wide openings between these meri- 

 steles will allow of free communication between the 

 cortex and the central pith, which in such stems is 

 often very bulky. 



Fig. 140. Vascular skeleton 

 prepared by maceration and 

 dissection from the stock of 

 Diyopleris. (After Reinke.) 



(X2.) 



Perforation 



In addition to the foliar gaps there are still 

 other means of communication between cortex and 

 pith in many Leptosporangiate Ferns of advanced 

 type. " Perforations " of circular, oval, or elongated 

 outline are found, and they are specially a feature 

 of rhizomatous Ferns where the leaf-gaps would 

 only occur at intervals. Sometimes they are im- 

 perfect, appearing as xylic-perforations, with or 

 without involution of the endodermis : sometimes 

 an isolated perforation may occur, as in Metaxya: 

 but commonly they are so numerous that in a transverse section the 

 meristeles appear as a ring of small isolated tracts, each surrounded 

 by its complete endodermal sheath. Perforations were demonstrated by 

 Mettenius in Stenochlaena temcifolia (Desv.), Moore (Fig. 141), and in many 

 other rhizomatous Ferns. The typical solenostelic structure is usually 

 present in relatively primitive creeping stems. But in them the continuous 

 stelar tube would form an obstacle to gaseous interchange between cortex 

 and pith. The presence of " perforations " in creeping Ferns in other re- 

 spects of relatively advanced type may be held as an amendment on that 

 structure, whereby gaseous interchange is ensured. At the same time, as in 

 the lattice leaves of the flowering plant Ouvirandra, an increased proportion 

 of surface to bulk of the vascular tracts is secured, a point which will be taken 

 up again in Chapter X. 



In the advanced Leptosporangiate Ferns the ascending or upright stem has a vascular 

 system in the form of a tubular lattice-work, embedded in a matrix of "ground-tissue." 

 It has been seen from evidence, both ontogenetic and comparative, that this is derived from 

 the expansion and disintegration of a protostele. The origin of the internal ground-tissue 

 or pith has here been attributed to change of procambial destination. In Gwynne-Vaughan's 

 words, a " theory of transformation " has been adopted. Others have suggested an intru- 

 sion of cortical tissue from without, which may be described as a " theory of substitution." 



