The Structure of Stems 133 



by soft layers of cork cells or by layers of sclerenchyma. The 

 pith and the inner part of the cortex are made up of parenchyma. 

 In annuals and young perennials the cortical parenchyma con- 

 tains chlorophyll and resembles the mesophyll of the leaf in 

 appearance and function. It is this tissue that forms the inner 

 '' green bark " of twigs and gives the green color to the stems and 

 branches of herbaceous plants. 



There are, then, four distinct layers in dicot stems : (i) on the 

 outside is the epidermis ; (2) from the epidermis to the bundles 

 is the cortex; (3) inside the cortex is the bundle-cylinder; 

 (4) the pith forms the axis of the stem, filling the space inside 

 the cylinder of bundles (Fig. 75). 



Between the bundles of the dicot stem there are strands of 

 parenchyma cells that connect the pith parenchyma with the 

 cortical parenchyma. These are the pith rays. They convey 

 food across the stem, and with the other parenchyma cells form 

 a complex tissue system in which foods accumulate and from 

 which they later move to other parts of the plant. 



General structure of the dicot bundle. The bundles in a plant 

 stem are continued above in the veins of the leaves, and below 

 in the bundles of the roots. In the dicot stem these bundles 

 contain four tissues; (i) the water-conducting tissue, (2) the 

 food-conducting tissue, (3) the cambium, and (4) the mechanical 

 tissue. The cambium is a layer of thin-walled cells that hes 

 lengthwise in the bundles and separates the water-conducting 

 tissue from the food-conducting tissue. 



The water-conducting tissue contains long, tube-hke vessels 

 made up of cylindrical cells joined end to end, often for consider- 

 able distances without end-walls between them. These tubes 

 (tracheae) usually have heavy walls marked by spiral and lattice- 

 form thickenings. When mature they are empty of protoplasm. 

 In other words, they are the coverings of dead cells joined to- 

 gether, forming tubes usually several inches, more rarely several 

 feet, in length. Mixed with them are smaller and shorter tubes 



