On Descriptions of Vascular Structures. 
109 
becomes replaced by other tissues—ultimately by central parenchyma 
bounded externally by endodermis, between which and the (now 
annular) xylem there is a ring of phloem ; at the nodes the central 
parenchyma becomes continuous with the cortex through the leaf- 
gaps, which finally become crowded, so that, at any succeeding 
level, the central tissue and the cortex are connected at more than 
one point. This mode of description may he taken to imply that 
the central tissues within the xylem-ring of the seedling belong to 
the stele. Whereas, if we trace the series in the reverse order, we 
start with vascular strands arranged in a ring and separated from 
one another by leaf-gaps; the cortex is similar to the central 
parenchymatous tissue and continuous with it, and the two would 
naturally be described as parts of one tissue (cortex or ground- 
tissue). Hence the downward continuation of the centrally placed 
parenchyma, which we find lower down within the closed vascular 
ring (and internal to an endodermis) we should continue to designate 
as cortex or ground-tissue, not as part of the stele. 
Thus our opinion of the morphological nature of this tissue, 
i.e., as to whether it represents stelar tissue or cortex, may he first 
suggested simply by appearances, which differ according to the 
direction chosen for tracing the tissues, and apart from any further 
reasoning. In these provisional opinions our view is essentially 
based on the position of the tissue, if one follows the acropetal 
order, and on its continuity with and structural similarity to the 
cortex 1 , if one adopts basipetal sequence. 
For purely descriptive purposes tissues may he traced upwards 
or downwards, according as the one or the other method is found 
to he more convenient for each individual case, hut it has to be 
borne in mind that, if any claim to morphological treatment is 
intended, the phylogenetic history of the tissues concerned must be 
enquired into, and either a separate theoretical statement with 
regard to their morphology must he made, or the topographical 
statement must be re-worded according to the view arrived at as to 
the first origin of these tissues. 
The difficulty in obtaining a criterion of the morphological 
1 Continuity and structural identity have sometimes been 
treated as proving morphological identity. This cannot be 
accepted, but the different parts of the protostele are here 
assumed to have the same morphological value, and the same is 
supposed for the different vascular parts of the dictyostele, because 
a continuous conducting system has no doubt been retained con¬ 
stantly throughout the stem. 
