338 SCIENCE PROGRESS 



The Morphological Status of the Pith 



In the majority of cases the first appearance of this tissue is 

 an aggregation of parenchymatous elements within the hitherto 

 solid rod of xylem of the protostele. This parenchyma may be 

 directly continuous with the same tissue of the siphonostele. 

 Now if the development of the vascular system stopped here 

 there would probably be very little difficulty in accepting the 

 pith as a morphological tissue ; but development goes further, 

 the siphonostele gives rise to the dictyostele, which widens out 

 considerably, enclosing in its network a mass of parenchyma — 

 sometimes also sclerenchyma — which is indistinguishable from 

 the more externally placed cortex. Where, in such cases, does 

 the pith end and the cortex begin ? Again, in many cases, e.g. 

 Angioptcris, Marattia (Farmer and Hill, 20), Dancea (Brebner, 8), 

 Matonia (Tansley and Lulham, 61), and other plants, this " pith " 

 is traversed by vascular strands. Further, vascular tissue may 

 appear in the external ground tissue ; Brebner (8) recorded a 

 case in Dancea where a tracheid occurred in the ground tissue 

 of the leaf-bases, which helps "to show that the line to be drawn 

 between stelar and extrastelar tissue is not a hard and fast one." 

 Hence, numerous difficulties have to be surmounted before many 

 botanists can accept the tissue enclosed by the vascular cylinder, 

 in all its varieties, as having any morphological significance. 



Jeffrey argues that inasmuch as the internal phloeoterma, 

 when present, is in direct continuity with the external 

 phloeoterma 1 through the leaf-gaps, it is therefore of the 

 same value morphologically, and hence it follows that the 

 pith is not part of the stele, but represents a part of the cortical 

 ground tissue ; in other words, it is extrastelar. 



Boodle (5, p. 533) considers that inasmuch as, in all pro- 

 bability, the solid protostele was the primitive type which 

 gave rise to the more complicated forms of vascular systems, 

 it follows that "whatever tissue is found within the xylem is 

 presumably morphologically stelar. Assuming an exarch 2 

 protostele, a pith may have originated by incomplete differ- 



1 It should be mentioned that Jeffrey uses the term phlceoterma synonymously 

 with endodermis. 



2 A term in common use to denote the position of the first-formed element, 

 the protoxylem, of the wood. Thus, the protoxylem is exarch when it is situated 

 in a peripheral position ; the metaxylem, or later-formed wood, is developed 

 centripetally, and is thus more internally placed. 



