146 THE INDIVIDUAL ORGANISM 



the cambium. The cells that lie between the arms of the xylem star and 

 the phloem strands (and which become parenchyma in the roots of annual 

 plants) in perennials retain their meristematic properties and constitute 

 a cambium layer. The cambium forms a fluted cylinder (a wavy circle in 

 cross section), extending out around the ends of the xylem star and 

 separating the xylem from the phloem strands, so that the xylem is inside 

 the cambium and the phloem is outside. Like the cells of the growing 

 point, those of the cambium layer are undifferentiated and are capable 

 of rapid growth and division. In the region between the xylem arms and 

 the phloem the cells cut off on the side toward the xylem become xylem; 

 those cut off on the side toward the phloem become phloem. Where the 

 cambium layer bends around the arms of the xylem star, it produces 

 neither xylem nor phloem but only parenchyma cells. 



As this growth process goes on, the cambium ring (or cylinder) smooths 

 out. Between the arms of the original xylem star there grow out masses 

 of secondary xylem, separated by rays of parenchyma opposite the arms 

 of the star. The original phloem is pushed out toward the periphery of 

 the growing root and may be crumpled up and destroyed by the pressure 

 of the developing secondary phloem and xylem. The secondary phloem 

 lies just outside the masses of secondary xylem, separated from them by 

 the cambium ring. 



While this thickening has been going on, the pericycle (also composed 

 of parenchyma) has again taken on the functions of meristem and has 

 formed a second cambium layer (the cork cambium) outside the first. 

 This cambium produces a layer of cork cells, the walls of which are 

 impervious to water because they contain suberin. The cork layer cuts 

 off the cortex from access to food and water, causing it to die and dis- 

 appear. Thus the older parts of the root come to consist solely of the 

 greatly modified stele, with a covering of corky bark instead of the original 

 epidermis. 



THE FUNCTIONING OF THE ROOT 



The soil as the environment of roots. The soil consists of a porous 

 mass of large and small mineral grains, together with a variable propor- 

 tion of organic material derived from the decay of plant and animal 

 bodies and animal excreta. Its deeper parts are more or less saturated 

 with water, but in the upper layers, the spaces between the grains are 

 filled with air. In the minute passages and cavities of this aerated zone 

 live innumerable bacteria and simple plants, engaged in decomposing 

 the organic debris. Here also live minute animals of extraordinary diver- 

 sity, so numerous that their excrement and their dead bodies form impor- 

 tant constituents of the soil materials. In this zone, each mineral grain 

 is coated with a colloidal film of organic or inorganic material and en- 



