A Note on Vascular Tissue. 
197 
chyma must surely militate against the exaltation of the central 
cylinder to a rank of eminently different order from the external 
tissues. It is rather difficult also to understand the attitude adopted 
with regard to that portion of the leaf traces which traverse the 
cortex. Are they of cylindrical or extra-cylindrical nature ? 
In Dr. Jeffrey’s (9,10, 11) opinion, a very different view must be 
taken of the internal parenchyma in angiosperms and the more 
highly developed ferns. As the outcome of a long series of investi¬ 
gations on the development of seedlings in both classes, Dr. Jeffrey 
agrees with most other investigators in regarding as the primitive 
form of cylinder the solid rod of xylem surrounded by phloem and 
cut off from the cortex by an endodermis. Phloem is then said to 
appear within the rod of xylem, and after the appearance of a few 
leaf traces an internal endodermis arises, cutting off the internal 
phloem from a central parenchyma now appearing. This central 
parenchyma is continuous with the cortex through the leaf-gaps 
and is regarded as “intruded cortex”—a view diametrically opposed 
to that discussed above. The late formation of a dictyostele is 
simply owing to the overlapping of large leaf-gaps through which 
external and internal “ cortex ” are continuous. In the plant which 
has passed the seedling stage—particularly in the Phanerogams— 
the internal endodermis can no longer be detected. Nevertheless, 
Dr. Jeffreys regarded the internal parenchyma as being of the same 
nature as the external cortex. 
If the perhaps rather misleading term “intruded cortex” be 
replaced by the term “ ground parenchyma ” or “ ground tissue ” 
(to allow for the frequent development of sclerenchyma) we 
arrive at the conception of a ground-tissue system through which run 
the vascular strands. In the most primitive case the vascular 
system is a solid strand, in others a hollow cylinder, enclosing 
ground tissue; in others again it is a network of conducting strands (6). 
This offers a very simple and natural method of regarding the 
vascular system of plants, and entirely obviates the necessity for 
any abstract distinction between vascular tissue, potential-vascular 
tissue, and non-vascular tissue which likewise may develop vascular 
tissue, but which does not belong to the vascular system. 
If Dr. Jeffrey’s results be regarded from a physiological point 
of view the distribution of the endodermis as described by him 
assumes a perfectly natural aspect. It cuts off vascular from non- 
vascular tissue, and no abstract consideration necessitates the 
postulation of a theoretical boundary between the extra-cylindrical 
and intra-cylindrical ground tissue such as is demanded by the other 
