iS 



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 

 and more 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, ancl 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 



