634 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VoL. VI. 
4. Van Tieghem’s three types of central cylinder indicated above are 
all merely modifications of a single type, which has been designated 
by the writer, siphonostelic. In this type the central cylinder is 
primitively a fibro-vascular tube with foliar lacunz opposite the points 
of exit of the leaf-traces. In the so-called polystelic modification, the 
central cylinder has internal, as well as external phloém, and may be 
described consequently as amphiphloic. In the astelic type of axis so- 
called, the internal phloém is absent and the central cylinder is accord- 
ingly 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. 
5. The siphonostelic type of central cylinder as defined above is 
probably to be regarded as the result of the mechanical strengthening 
of the cauline axis to enable it to support the palingenetically 
large leaves which are characteristic of the Angiospermz, Gymno- 
sperme, and Filicales. In these three groups, the siphonostelic, fibro- 
vascular cylinder is invariably distinguished by the presence of gaps 
corresponding to the points of exit of the leaf-traces, and, in this fea- 
ture, offers a marked contrast to the tubular central cylinder of the 
Lycopodiales and Equisetales, in which there are no foliar lacune, but, 
on the contrary, gaps subtending the branches. 
6. A study of the development and structure of the fibro-vascular 
apparatus of the various groups of vascular plants is likely to throw 
considerable light on their phylogeny and to elucidate the causes of the 
morphological differences in the structure of the central cylinder of root 
and shoot. 
For many kindnesses in the matter of supplying material for this 
research, the writer wishes to express his obligations to the Director of 
the Royal Gardens, Kew; to the Director and Assistant Director of 
the Botanical Gardens of Harvard University; to Dr. D. H. Scott, 
Honorary Director of the Jodrell Laboratory, Kew, and to Mr. Jackson 
Dawson, of the Arnold Arboretum of Harvard University. He is 
specially indebted in this respect to Prof. G. L. Goodale, Director of the 
Botanical Gardens of Harvard University. Lastly, the writer owes not 
a little to his assistant, Mr. R. B. Thomson, B.A., for help in securing the 
seedlings necessary for the present investigation. 
