STELAR THEORIES 331 



type, which is the siphonostele, i.e. " the central cylinder is 

 primitively a fibro-vascular tube with foliar lacunae opposite the 

 points of exit of the leaf-traces." He further remarks that 

 " in the so-called polystelic modification, the central cylinder 

 has internal as well as external phloem, and may be described 

 consequently as amphiphloic. In the astelic type of axes 

 so-called, the internal phloem is absent and the central cylinder 

 is accordingly to be designated ectophloic. The medullated 

 monostelic type of Van Tieghem is derived from the last-named 

 by the degeneration of the internal phlceoterma or endodermis " 

 (Jeffrey, 37, p. 38). 



Thus, fig. 4 c represents the amphiphloic condition, inasmuch 

 as the phloem completely surrounds each strand of xylem ; the 

 ectophloic type is similar, but the distribution of the phloem 

 is not so extensive — instead of completely encircling the wood 

 it is developed only on the outer margin of each strand. 



The essential facts on which these conclusions are based 

 have been amply verified by the work of Boodle (2 — 5), Brebner 

 (8), Gwynne Vaughan (25 et. seq.), Farmer and Hill (20), Leclerc 

 du Sablon (42), Tansley (57 et. seq.), and others. 



The second factor which has an important bearing upon 

 the original hypothesis is, whether the endodermis really is the 

 innermost layer of the cortex. 



Not a few instances have been noted where the endodermis 

 and pericycle have seemingly arisen from the same initial layer 

 of cells, and not only these two tissues, but others. Thus 

 Boodle (3, p. 377) remarks: " In Schizcea digitata a comparison of 

 transverse sections of the young stem at different stages of 

 development points to a possibility that the endodermis, peri- 

 cycle and phloem may be formed from the subdivision of a 

 single layer." Again, Tansley and Chick (58) have shown that 

 in the case of Schizcea malaccana the endodermis, pericycle, and 

 the vascular ring all arise from a single initial layer. 



These instances point to the fact that in many cases the 

 endodermis is not the innermost cortical layer. 



If a median longitudinal section through the stem-apex of 

 a plant be examined, there may sometimes be made out three 

 fairly distinct layers of cells : an outermost stratum consisting 

 of a single layer of elements and termed the dermatogen ; 

 immediately below is a second series of cells, the periblem ; and, 

 finally, the central core, which is designated the plerome. These 



