CHAP, ii The Differentiation of the Tissues 211 



and polystelic forms. Where the lateral fusion of the 

 conjunctive tissue round the bundles in the gamostelic type 

 is complete, he spoke of them as gamodesmic. 



In 1890 Van Tieghem extended his astelic type to include 

 Equisetum and Ophioglossum, regarding the apparent mono- 

 stelic condition of some species of the former as deceptive, 

 owing to the differentiation of the cells of the inner endo- 

 dermis being so slight that the layer has become unrecog- 

 nizable histologically. 



Strasburger discussed Van Tieghem's position in 1891 

 and showed that on the whole he agreed with his concep- 

 tion. He suggested that as the astelic condition arises 

 from the separation of the bundles of an original monostele, 

 the term schizostely would be a preferable term to desig- 

 nate it. As the endodermis shows so much variety in 

 histological appearance, he proposed the name phleoterma 

 as a substitute for the older name. 



Van Tieghem proposed the term meristele for such 

 branches of the stele as can be traced into the petiole. 

 He claimed that they are derived from part only of the 

 stele of the stem. Strasburger suggested schizostele again 

 as preferable to indicate this relationship. He denied that 

 they could appropriately be called monosteles, on the ground 

 that their pith is continuous with the cortical parenchyma 

 of the axis. 



In 1893 an important paper was published by Plot, 

 which had a bearing upon the differentiation of the mono- 

 stele and the nature of gamostely. He investigated a large 

 number of plants, principally Dicotyledons, and claimed 

 to have discovered in them all a special ' perimedullary ' 

 zone separating the vascular bundles from the pith, and 

 corresponding in the main to the pericycle on their 

 other side. This zone he found to correspond with the 

 central cylinder in the way in which the divisions of its 

 cells took place, and to differ radically from the pith in 

 this particular. Tracing it upwards to the apex, he found 



02 



