certain Species of the Genus Christisonia. 1 1 7 
or of considerable size, having usually very dense proto- 
plasmic contents in which occur many large vacuoles. The 
mode in which they are cut out of the sieve-tube often gives 
that element a peculiar bent and curved appearance. There 
is frequently an appearance as if a companion-cell was cut 
off on each side of the same sieve-tube ; in some, these three 
cells are equal in length, the two lateral ones having denser 
contents than the middle one ; in others, the middle cell is 
longer and more trumpet-shaped, overtopping the two lateral 
ones at either end and expanding above them. This suggests 
that the middle segment of the original procambial cell has 
grown in length to become the sieve-tube, while the two 
sister-segments remain behind as companion-cells. The 
sieve-tubes are often arranged in regular radial rows for some 
distance. 
The parenchyma-cells are wide, elongated elements. 
The xylem- elements are usually very short, with transverse 
or sometimes slightly oblique terminal walls ; often they are 
more elongated. They have reticulate thickenings. Their 
course is the same as that described in the last species. On 
the outside, nearest the phloem, are the protoxylem-elements, 
which are more elongated than the others, while their 
thickenings are partly spiral, partly reticulate ; they are not 
very typical for protoxylem, but are rather transitional in 
character between this and the later-formed elements. The 
perforations in the terminal walls of the vessels are in this 
plant distinctly seen in sectional view. 
The outermost stone-cells , scattered in the cortex, are short 
and more isodiametric in contour, and are isolated. Those 
immediately surrounding the central cylinder are usually 
very much elongated, thick walled, and massed together in 
groups. 
In some places the roots show a slight swelling, which is 
the commencement of the large, thick, tuberous portions 
of the root which are so common. 
These tubers occur at frequent intervals in the root and are 
very characteristic of this species. If a section be made 
