654 
Proceedings of Royal Society of Edinburgh. [sess. 
The endodermis, of cells smaller than those of the cortex, is 
fairly well marked, and the thickenings on the radial walls are 
well stained with aniline safranin. 
When secondary thickening begins the cambium appears first 
in the small celled tissue opposite the primary phloems, and 
secondary xylem is formed centrifugally in this position. The 
cambium thus forms a ring broken by the four xylems. At a later 
stage the pericycle cells outside the protoxylems divide tangenti- 
ally to form a cambium which bridges across the gap. These 
divisions do not take place in the cells immediately outside the 
protoxylem, and there are usually a few crushed pericycle cells 
to be seen between it and the secondarily formed tissues. The 
cambium ring is now complete, and forms secondary xylem and 
phloem normally. 
Following the root, upwards, near the collet certain cells of the 
pericycle opposite the phloems divide to form a cork cambium. 
Passage from Root to Hypocotyl. 
The passage from the root structure to that of the stem takes 
place in the space of a few millimetres in the swollen root portion 
mentioned above. A section at the collet shows eight typical stem 
bundles with their protoxylems directed towards the centre ; and 
considering everything above the collet as stem and that below 
as root ; the transition from root to stem takes place entirely in 
the root. 
The plant from which the following observations were made 
was young, the cotyledons being still in the seed ; the root was 
rather more than 4 cm. in length, and the passage occupied about 
9 mm. 
Passing upwards to this region, the four xylem bundles, which 
in the unthickened root are narrow and conical (fig. 1), become 
gradually broader at the base and of less radial depth (fig. 2). 
At length the four bases form a nearly continuous ring round 
the pith, and the protoxylems are drawn down to the middle of 
each bundle. The appearance at this stage is that of eight xylem 
bundles lying with their long axes directed tangentially and joined 
two and two by their protoxylems (fig. 3). The four phloem 
