NATURE OF PLANTS 



59 



and the cells are changing in character, so that they perform 

 different duties. This is illustrated in the cross section of the 

 root (Fig. 36), where we see that the outer cells have become 

 modified, forming an epidermis with root hairs 

 for absorption. Within the epidermis is a broad 

 zone of cells, the cortex, often used for the stor- 

 age of manufactured foods, while in the center 

 of the stem are the vascular bundles. The woody 

 portion of the bundle, or xylem, radiates out- 

 ward from the center and the soft portion, or 

 phloem, alternates with it (Fig. 36, x, p). The 

 materials absorbed from the soil are largely 

 transported up to the stem and leaves through 

 the xylem, and the foods manufactured by the 

 leaves reach the root by means of the phloem 

 cells. The branches of the root originate in a 

 very curious way from the cells just outside the 

 xylem. These cells by repeated divisions form 

 lateral roots which gradually destroy the tissues 

 in their way and finally grow out to the surface 

 of the root (Fig. 37). By this arrangement they 

 are provided with a root cap and fully prepared 

 to enter the soil on emerging from the old 

 root. 



2S. The Transport of Water in the Root. — ^The inner layer of 

 the cortex, the endodermis (Fig. 36, end), consists of a layer of 

 cells which, in the older part of the roots, forms a very compact 

 and more or less cutinized ring of cells. This tissue is supposed 

 to function in preventing an undue loss of water from the con- 

 ducting strands of xylem to the cortex. It should be stated here 

 that the root hairs not only absorb fluids but also force out the 

 absorbed substances into the adjoining cells of the cortex. All 

 living cells have this power of absorbing and expressing fluids. 

 So it comes about that the absorbed fluids are forced by the root 

 hairs into the cortical cells and by them they are passed on to the 

 cells of the xylem when by some force not known they are drawn 

 up the stem into the leaves. The force exerted by these myriad 



Fig. 33. Dia- 

 gram of a section 

 taken through 

 the center of a 

 corn root : r, root 

 cap ; e, epider- 

 mis; i;, cortex; p, 

 central region. 



