408 BOTANICAL GAZETTE [DECEMBER 
In 1897, Dr. E. C. Jeffrey put forward another view of the 
vascular system, based upon a study of the young sporophyte. 
Here, too, the stele is the unit. According to this conception 
there are two primitive types of vascular axes; the first the same 
as Van Tieghem’s primitive type, and designated “ protostelic ;” 
the second, one in which there is a hollow cylinder, or ‘siphono- 
stele,” whose external wall abuts on the cortex, and whose 
internal wall encloses the medulla, and which possesses internal 
as well as external phloem. This is the ‘‘amphiphloic siphono- 
stelic” type, called by Van Tieghem the ‘‘polystelic.” The 
commonly called “‘astelic’’ modification results from the amphi- 
phloic type by a degeneration of the internal phloem, and the 
medullated monostelic type of Van Tieghem is derived from the 
astelic by the loss of the internal phloeoterma or endodermis. 
A study of development from the seedling is likely to show how 
these and other modifications in the stellar structure have been 
derived from the primitive types. Attention is also called to 
certain portions of the wall of the siphonostele in which the 
vascular tissues do not develop. These places lie above the 
points of exit of branch traces, and of leaf traces, and are known 
as ramular and foliar gaps respectively. Through these gaps the 
tissues outside and inside connect. In transverse section, the 
connecting tissues seen constitute the medullary rays, and the 
segments of the woody cylinder with adjacent phloem and 
parenchyma the bundles. A fact of great phylogenetic impor- 
tance in dealing with ‘“‘gaps” was further pointed out, namely, 
that in small leaved plants, as in the Lycopodiaceae, Equise- 
taceae, etc., only ramular gaps occur. These plants are grouped 
in the division Lycopsida, and their steles are said to be 
cladosiphonic. In all other vascular plants there is a gap for 
every leaf. These constitute the large leaved plants, the 
Pteropsida, and their steles are said to be phyllosiphonic. 
As a matter of theory, it is suggested that the siphonostele 
arose from the protostele for mechanical causes, in the Lycopsida 
to support the branches, and in the Pteropsida to support the 
3©JEFFREY: Trans. Brit. Assn. Toronto. 1897. 
