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This is very clearly seen in the arrangement of the leaves 
on the horizontal branches of a tree, or in the case of ivy 
growing up a tree trunk or on a wall. Even in plants 
possessing no stem, and in which therefore the leaves are 
found close to the ground forming a rosette, the leaves 
are found to overlap very little and particularly when they 
are stalked it can be clearly seen that young leaves which 
are formed near the middle of the rosette do not overlap 
the older ‘leaves, as the latter become pushed out more 
soe pe from the centre by the elongation of the leaf- 
stalks. 
The function of the stem being to bear and display 
the leaves it follows that, to carry the foliage and to resist 
the strain which winds will exert upon leafy plants the 
stem has to meet certain mechanical requirements. The 
rigidity, and at the same time the elasticity of the stem 
is attained by the development of certain thick-walled 
cells which collected into groups are so arranged as to 
give the greatest amount of strength with the smallest 
expenditure of material. These strengthening cells are 
displayed in plants on the principles which have been 
adopted by engineers in the manufacture of rigid, and at 
the same time elastic structures. We have, firstly, the 
hollow cylinder as is seen in grass-haulms. The slender 
grasses can maintain their erect position by virtue of the 
mechanical properties of their straw. The bamboo cane 
is perhaps the ‘most powerful of all in this form of con- 
struction. In other cases we find the stems developing 
internally a system of girders which give them the strength 
they require. 
There are some plants in which the stem has not 
sufficient rigidity to grow erect as it does in most instances. 
In such cases the stem may trail along the ground, often 
covering a large area and the plant is then often modified 
as a shade plant unless it grows among very short vege- 
tion, as in the case of some mountain plants. Sometimes 
by using the support of the rigid stems of other plants 
twiners and climbers are able to reach the light though 
they have to compete with much stouter vegetation. 
Twiners, such as the scarlet runner and the hop, have no 
special climbing structures, but their slender stems ending 
in a heavy terminal bud are bent over to one side, and on 
close examination can be seen to rotate very slowly, either 
clockwise or counter clockwise. This circular movement 
