igig] MILLER— CYCAS MEDIA 209 



cal cylinders diminishing in the thickness of their wood and phloem 

 as they neared the tip of the stem, until finally they disappeared 

 entirely, the outermost disappearing first and the innermost last. 

 Protoxylem, at least of the spiral kind, he found to be entirely 

 obliterated in the normal cylinder, although it seems to have been 

 of frequent occurrence in the leaf traces, where the crushing pres- 

 sure of thickening cells was less effective. He mentions no proto- 

 xylem in connection with the supernumerary cylinders, either as 

 to its presence or absence, nor in connection with leaf traces 

 coming from these cylinders. 



Scott's^ work in 1897 led him to the conclusion that the polyx- 

 yhc habit was a derivation from the habit of ancient stems among 

 the CycadofiJicales which developed layers of concentric bundles, 

 for example, Medtdlosa. 



WoRDSELL" again in 1900 pubUshed an account of the seedling 

 stem structure in Bowenia. There he found beginnings of a super- 

 numerary vascular cyHnder outside the normal one. Hints of 

 concentric bundles, which he found in Bowenia and earlier in 

 Macrozamia, led him to agree with Scott in the idea of the phylo- 

 genetic origin of supernumerary cylinders. 



Coulter and Chamberlain^ published in 19 10 a summary of 

 previous investigation pertaining to the vascular anatomy of cycad 

 stems, and in addition gave a short description of the gross topog- 

 raphy of the polyxylic habit. 



Jeffrey's^ work, pubUshed in 191 7, is the most recent account 

 of this cycad peculiarity. To him also it is apparent that super- 

 numerary cylinders arise in the pericycle. He objects, however, 

 to Scott's conclusions in regard to the phylogenetic origin of these 

 vascular cylinders; he believes rather that they are a result of an 

 ancient climbing habit of the stem. Such situations, he says, are 

 frequent in numerous cHmbing stems of the present time, stems of 

 both gymnosperms and dicotyledonous angiosperms. 



3 Scott, D. H., The anatomical characters presented by the peduncles of Cycada- 

 ceae. Ann. Botany 11:399-419. 1897. 



^WoRDSELL, W. C, The anatomical structure of Bowenia speclabilis. Ann. 

 Botany 14:159-160. 1900. 



5 Coulter, J. M., and Chamberlain, C. J., Morphology of gymnosperms. 1910. 



* Jeffrey, E. C, The anatomy of woody plants. 191 7. 



