STEMS ' 101 



132. Arrangement of rtieclianical tissues in a 

 petiole. — The types of stems described above stand 

 erect and support the weight of the leaves like a 

 pillar, and the bending force of the wind like a 

 tower. The petioles of leaves generally hold the 

 blade in a horizontal position, and the weight acts 

 always to bend the petiole in one direction, down- 

 ward. To meet this strain it is necessary to have 

 the building material arranged in a half circle, as 

 may be seen if the leaf of the maple or chestnut is 

 examined. 



133. TJie firmness of plants that "become limp 

 when dried. — Select a young stem of any plant which 

 would become limp if you laid it in the sun. The 

 tips of elder stems in April or May will offer splendid 

 material. Cut away from opposite sides of the stem 

 until only a thin strip remains, which includes the 

 central part of the stem. Now divide this exactly 

 down the middle with a sharp knife. This will 

 make two sheets of material, each of which is com- 

 posed of a strip of living pith full of water under 

 pressure, and the wood which will bend but not 

 stretch. After a few minutes place the two strips 

 together in their original position. What changes 



