VARIATIONS IN ROOT-STRUCTURE 69 



except for the radial walls, which persist unaltered, implying 

 that they are chemically different from the other walls of the 

 endodermal cells. These features of the radial walls are com- 

 monly exhibited by the endodermis of roots. 



The purpose of the endodermis is still obscure, but such a 

 layer, showing the characteristic thickening of the radial walls, 

 is found in the roots of many Flowering Plants, as well as in the 

 stems of aquatics (cf. p. 171). The firm lateral connection of the 

 cells due to the thickening renders this layer an efficient sheath 

 to the vascular system. The considerable thickening of the 

 entire endodermal wall which obtains in some cases (cf. below), 

 suggests a probable mechanical value. In the unthickened state 

 the chief function of this layer may perhaps be to cut off the 

 water-conducting strand from the air-spaces of the cortex, a 

 delimitation rendered necessary by the frequent negative pressure l 

 within the vascular system. Not uncommonly the layer of 

 cortical cells next to the endodermis develops characteristic 

 thickenings on the transverse and radial walls, a -feature well 

 seen ill the roots of many Cruciferae (e.g. White Mustard), and 

 no doubt of mechanical value. 



The pericycle which lies immediately within the endodermis 

 is another continuous layer not characterised by any structural 

 peculiarities (Fig. 27, p.). 



Many of the features just described are typical of roots 

 generally, viz. the aggregation of the vascular tissue near the 

 centre ; the alternation of phloem and xylem resulting in a radial 

 structure ; the peripheral location of the protoxylem ; the wide 

 cortex ; the differentiation of the endodermis ; and the occurrence 

 of exodermis and piliferous layer. Roots differ among one 

 another in two principal respects namely, as to the number of 

 phloem- and xylem -strands, and in the presence or absence of 

 parenchyma (pith) in the centre of the conducting tissue. As 

 regards the former feature, there may be two or more of such 

 strands, roots being described as diarch, triarch, tetrarch (Fig. 27), 

 pentarch (Fig. 30), etc., according as the number of alternating 

 xylem- and phloem-groups is two, three, four, or five, etc., whilst 

 when they are numerous the structure is said to be poly arch 

 (Fig. 29). Thus the root just examined is tetrarch, that of the 

 1 Cf. F. & s'., p. 121. 



