THE ROOT 141 



termed the pittferous layer : it is directly concerned with the 

 important work of absorbing watery solutions from the soil. In 

 a transverse section (2, Fig. 72) taken at a point not far away 

 from the extreme end of the root, many of the cells of this layer 

 are seen to be much elongated ; these are the root-hairs, pre- 

 viously mentioned in chapter iii. The cell-walls are all thin and 

 uncuticularised and are readily permeable to water, thus differing 

 essentially from the epidermal cells covering the parts above 

 ground. 



Immediately beneath the piliferous layer is the cortex (i), which 

 is continuous with the same ground-tissue in the stem. The cells 

 of the cortex are usually parenchymatous and thin-walled with 

 many intercellular spaces between them ; chloroplasts are fre- 

 quently absent, hence the pale colour of most young roots. 



The innermost layer of the cortex, or the endodermis (e), is 

 generally very distinct in roots. Its cells are closely united with 

 each other in the form of an uninterrupted circle, an arrange- 

 ment which effectually prevents the leakage of gases from the 

 intercellular spaces of the cortex into the water-conducting tissues 

 of the central cylinder. The transference of water from the root- 

 hairs and cortex through the endodermis into the conducting 

 tissues of the central cylinder is, however, not interfered with. 



In most roots the central cylinder is of smaller diameter, and 

 contains less parenchyma than that of the stem, although one is a 

 continuation of the other. It is, however, in the disposition of 

 the tissues within the central cylinder that the most important 

 differences between stems and roots are seen. 



The pericycle («), like that of a stem, may consist of a single 

 layer or several layers of cells. From this internal tissue arise all 

 lateral secondary roots, which must therefore necessarily bore 

 their way outwards through the surrounding cortex before they 

 become visible on the outside of the root (see Fig. 9). The 

 wood (w) and bast {d) portions of the vascular bundles, instead 

 of being conjoined as in a stem^ are arranged alternately side by 



