STELAR THEORY 189 
influenced the thoughts of Van Tieghem, and stimulated him in the 
direction of his later generalisation; but it may be remarked that what 
Sachs did in urging the integral view of the shoot on more general 
grounds, Van Tieghem, by his introduction of the stelar theory in place 
of the mere study of the individual vascular strands, did on a basis of 
anatomical investigation. Both of these reforms tended in the same 
direction, viz. towards the conception of the shoot as a whole, with axis 
and leaf as its constituent parts. It may be said that any step of 
observation or of ‘reasoning which tends to emphasize the primary indi- 
viduality of the leaf, leads towards some phytonic theory of the shoot at 
large: any step which tends to emphasize the primary individuality of 
the axis leads towards some strobiloid view. The effect of the stelar 
theory of Van Tieghem has been in the latter direction. The recognition 
of the vascular column of the axis as a structural unit of the conducting 
system has gone far towards the reinstatement of the axis on the basis 
of structure, as a substantive and essential part of the shoot; and the 
change of view has been in opposition to those phytonic theories which 
would regard it as a mere congeries of leaf-bases. It will accordingly be 
important to consider this matter carefully in its bearings on the general 
theory of the shoot in the sporophyte. 
Van Tieghem recognised the central cylinder of the axis in the great 
majority of plants as an anatomical region coordinate with the anatomical 
regions of the cortex and the epidermis, which lie outside it: he designated 
it the stele. This cylinder is delimited by certain continuous sheaths: the 
inner, the pericycle, belongs typically to the stele: the outer, the endo- 
dermis, belongs to the cortex: the boundary of the tissue held to be stelar 
is the surface between these contiguous layers. The stele thus defined 
consists of vascular tissue—xylem and phloem—and of conjunctive tissue 
—usually parenchyma. In certain plants throughout, and in certain regions 
of other plants, the structure of the vascular tissue of the axis is relatively 
simple and compact, consisting of a solid central core of xylem, with a 
peripheral band of phloem: this was probably the primitive or original 
type, though it may also result, as in some well-known cases, from reduc- 
tion: it is designated the protostele. But in very many shoots the type 
of stele is more bulky owing to the presence of parenchymatous tissues: 
the vascular tissue may thus be separated into distinct strands, and in 
that case they are usually arranged with a radial symmetry and embedded 
in the conjunctive parenchyma: this tissue occurs partly as the pith, which 
occupies all the central region, partly as a lateral and external packing for 
the several strands. Such a stelar structure, of either the compact or of 
the more bulky type, is found in the axis and in the root of the vast 
majority of sporophytes. 
Exceptional arrangements exist, however, in certain cases: the most 
important of these is the polystelic type of stem-construction, which is 
found in many Pteridophytes and in some few stems of Phanerogams. 
