STEMS 227 



stems are green and make the food for the plants. 

 They also, you recall, store water. 



Structure. An ordinary stem is a jointed structure. 

 In some, such as the corn stalk and the cane (used for 

 fishing rods), these joints, or nodes, are very evident. 

 In others, the joints are evident only because they are 

 the places where leaves and branches appear. If the 

 internal structure of a stem is studied in detail, it will 

 be found to be very complex and difficult to understand. 

 We shall attempt to study only such parts as can be 

 seen with the naked eye. If a twig of any growing 

 woody plant, such as the box elder or maple, be cut 

 across, it will be seen to be made up of four distinct 

 parts: 1. An outer protecting layer, the epidermis. 

 2. A second layer of soft tissue, usually green, 

 the cortex. 3. A layer 

 of wood, the vascular 

 cylinder. 4. A central 

 portion, the pith. The 

 cortex when green is 

 able to manufacture car- 

 bohydrates just as the 

 leaves do. The wood is 

 the conducting region, 

 and it also gives strength Fig . 94 . Cross section of a t hree-year- 



, . . ,., ,, old woody stem. 



and rigidity to the stem. 



If examined more closely this section of wood will be 

 seen to be divided into a number of segments by plates 

 of cells passing from the pith to the cortex. These 



