THE STELE THEORY 179 



the plerome of the stem, are formed, according to the 

 data in hand, outside the plerome, being derived, as is the 

 whole leaf, from the periblem and dermatogen, or from 

 the layers of meristem corresponding in position to 

 these." 



In the Bryophyta and Pteridophyta the growing 

 points were known to be occupied by a meristem which, 

 in the great majority of cases, could be traced to the 

 regular partition of a single apical cell, and attempts were 

 made to homologise the conditions existing in these 

 plants with Hanstein's layers as reported in Angiosperms. 

 Scott, in 1894, sums up for us the general conclusions 

 arrived at in the following words : " We may safely 

 conclude that, with the solitary exception of the origin 

 of the root cap, the first tangential cell walls do not 

 constantly mark out the main histogenetic layers from 

 which the principal systems of tissues are derived. Such 

 layers arise in different ways in different cases, and at 

 various distances from the apex." 



Seeing that Hanstein's classification of tissues on an 

 ontogenetic basis had not proved entirely satisfactory, 

 Sachs next formulated a physiological scheme, dis- 

 tinguishing epidermal, fibrovascular, and fundamental 

 systems derived from a uniform apical meristem, but he 

 also was adversely criticised by De Bary, who said 

 " it did not answer its purpose, which is to serve as a 

 basis for a uniform exposition of the various differentia- 

 tions of plant tissues." 



THE STELE THEORY 



The year 1870 saw the publication of an important 

 work by the French botanist, Van Tieghem, which laid 

 the foundation of an entirely new way of looking at the 

 architecture of the plant axis, and one which, though in 

 a modified form, holds the field to-day. This is what 

 you know as the " stele theory." Van Tieghem began 



