^6 Introduction to Botany. 



38. Protection against Drying. — As soon as the shoot 

 appears above the ground it is in danger of drying up, or 

 of becoming bruised or broken. To guard against drying, 

 the outer wall of the epidermis or exterior layer of cells 

 becomes infiltrated with a compound of fatty and waxy 

 substances known as cu^in, and so is^ rendered almost im- 

 pervious to water. Every one has noticed ho.w quickly an 

 apple shrinks and drys when its epidermis has been removed 

 by paring, and it would be a simple matter to demonstrate 

 that any young or succulent stem would quickly become 

 dry if its epidermis were stripped off. 



39. How Stems are Strengthened. — In order to strengthen 

 the stem, the walls of the cells in certain regions become 

 thickened, and sometimes woody, and the cells often be- 

 come elongated and more or less interlaced, as in the case 

 of wood and bast fibers. As evidence of the effectiveness 

 of wood and bast in strengthening stems, let the great elas- 

 ticity and strength of some of the well-known woods, such 

 as hickory, be called to mind, and the fact that hemp rope 

 and Hnen thread are made from the bast of plants. 



The stress which the strengthening elements must over- 

 come is produced usually by the wind and the weight of 

 the crown of the plant. The force of the wind bends the 

 stem, producing a stretching of the elements on one 

 side and a compression on the other, while the weight of 

 the crown produces a compressing effect simply. It is 

 clear that if a given amount of strengthening material be 

 distributed in the form of a hollow cylinder, all stresses of 

 the above nature can be overcome to the best advantage. 

 We find, accordingly, the strengthening elements of the 

 pericycle of Aristolochia arranged in this form, and the 

 same is true of the strengthening elements in the primary 

 cortex of corn, and of the wood and bast fibers in the 



