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. 



