certain Species of the Genus Christisonia. 107 
normal form. The xylem-elements of the root may be either 
trachcidcs or vessels; both kinds of elements occur indis- 
criminately, and are rather difficult to distinguish the one from 
the other. If the element be a vessel, the perforation may 
either occupy the whole surface of the terminal wall or the 
latter may be reticulately thickened, the perforations, which 
are of varying sizes, occurring between the reticulations. 
That tracheides, as well as vessels, are present, was inferred 
from the occasional occurrence of a delicate membrane stretch- 
ing across the terminal wall, its presence being indicated by 
the minute granules scattered about on the latter ; this was 
found between two xylem-elements, as well as where one 
of these abutted on a parenchyma-celk 
The phloem offers nothing specially worthy of remark ; 
it consists of sieve-tubes, companion-cells, and parenchyma. 
In longitudinal section the cells of the external layer appear 
narrow and elongated, and this is also the character of the 
outer cortical cells. The inner cells of the cortex are shorter 
and broader, and usually contain starch. Where there are 
stone-cells surrounding the central cylinder, these are of varied 
shapes ; they are usually short and rather narrow, sometimes 
quite square and broad, sometimes irregular in shape. The 
phloem parenchyma consists of broad, rather long cells, often 
filled with dense protoplasmic contents with large vacuoles. 
The sieve-tubes are well developed, their terminal walls are 
either transverse or slightly oblique ; the sieve-plate with its 
perforations is clearly distinguishable, and there is a well- 
marked callus-layer below. 
The companion-cells are numerous: they are either small 
elements, extending only part of the way along the sieve- 
tube, or they may be narrow elements, cut off along the 
whole mature length of the former. 
On the periphery of the cylinder are seen the long, spirally 
thickened protoxylem-elements, which are very narrow in 
diameter. The later-formed xylem-elements are usually 
quite short ; most of them show some amount of sliding- 
growth, as the terminal walls are often oblique. As might, 
