certain Species of the Genus Christisonia. 123 
near the centre of the stem ; four or five of these bundles may 
fuse into a kind of network near the base of the stem, then 
separate again into three or so, which almost directly after 
fuse with the central cylinder of the root which is running at 
right angles to the stem (Figs. 12, 12 a). 
In a longitudinal section the structure of the bundle can 
be easily seen. The xylem-elemen is are rather longer and 
narrower than those in the root ; they also undergo rather 
more sliding-growth, as their terminal walls are usually 
oblique, and more so than those of the root. The protoxylem- 
elements have usually a very loose spiral, which is often 
entirely separated from the wall and lies as an almost straight 
thread within the element ; others have annuli, which are 
rather distant and only just serve to keep the walls from 
collapsing ; these walls are frequently so thin as scarcely to 
be perceptible. 
The phloem-elements also undergo more sliding-growth. 
The sieve-tubes have a more oblique terminal wall than those 
of the root, and have several plates both on the terminal and 
the lateral walls. 
The cells of the ground-tissue are very loosely arranged 
and more or less contorted in outline, so that wide spaces are 
left between the cells. The reason for this may perhaps be 
sought in the fact that a properly-developed intercellular 
-system is necessary to ensure free access of air for respiration, 
and this may be correlated further with the absence of stomata 
on the stem ; it may also be to admit of easy and free trans- 
piration during the rapid growth which the plant undergoes 
in order to mature its flowers. As a result of the sliding- 
growth and contortion of the cells, the greater part of the 
common wall which connects any two cells, either laterally or 
terminally, splits at the middle lamella, and thus the two cells 
become separated. But some parts of the wall split more 
easily than others ; the parts which offer greatest resistance, 
and remain firm without splitting, cause each cell at that 
point to be drawn out into a tube owing to the rapid growth 
which each cell is undergoing; as a result of the stimulus 
