174 
THE PLANT WORLD. 
attachment between the two parts tends to set up strains in the 
inner tube, resulting in the puckering and contorting of the more 
delicate of the two structures. These points of retention are 
most often found in connection with the origin of a branch, either 
foliar or floral. In a few cases there were observed leaves or 
buds arise from the surface of the inner tube. As indicated above 
the inner tube tapers to a point which is usually free in the tubular 
cavity, and is about one-third the length of the stem from the tip, 
indicating something of the relative rate of growth of the two 
portions, supposing that the tubular structure of the stem began 
while the plant was still small. This would seem to be indicated 
by the slenderness of the tip of the inner tube, which was 
attached to the outer tube by the cellular tissue common to the 
two parts at that time as the upper ends of the two are now 
united by their common rim. 
The tip or rim of the stem is formed of a number of growing 
points, about one mm. apart, and rising about the same amount 
above the average level of the rim. The intercallary growth of 
meristem cells between the several growing points results in the 
tubular stem; this becomes a flat or ribbon stem when any one 
of the groups of intercallary cells ceases to act. The beginning 
