276 ME. B. G. COEMACK ON POLYSTELIC 



and transverse walls. Thus most of the endodermal cells are thickened on five walls, 

 but the usual thin- walled cells may he seen in the neighbourhood of protoxylem-groups. 



The vascular cylinder is normal, central, and, proportionately to the cortex, small and 

 concentrated, especially in comparison with thicker Areca-voots. Its conjunctive tissue 

 is an uninterrupted mass of sclerenchyma ; even the walls of the pericycle are mostly 

 thick and woody. 



The xylem- and phloem-strands have the usual radial arrangement. The xylem-groups 

 are single and l-shaped or paired, and V- or Y-shaped, with a phloem-group in the fork ; 

 the significance of this variation will be subsequently referred to. The youngest vessels 

 of the xylem-groups are large and constitute a conspicuous central ring. 



Thicker roots of Ai^eca, say about a centimetre in diameter, can be found, which are 

 practically normal in structure. In these the cortical zone is relatively less bulky than 

 in the thinner roots, and consequently the vascular cylinder is proportionately nearer 

 the periphery. The piliferous layer and enclosed layers of the cortex are essentially the 

 same as in the thinner roots. The cell-layers towards the periphery are thicker and 

 more obviously sclerotic. The sclerenchyma-fibres of the deeper cortex are grouped in 

 strong, definite strands, and the radial rifts are larger and more numerous. 



With the large size and peripheral position of the vascular cylinder is associated the 

 development of more numerous groups of xylem and phloem. These groups are larger 

 than in thinner roots, yet the difference in size is not sufficient to bring the youngest 

 vessels so near the centre. The conjunctive tissue is significantly different from that of 

 the thin root just described. It does not extend to the centre as a continuous scleren- 

 chymatous mass, but is so constructed as to leave a deeply-fluted central column, chiefly 

 of parenchyma. Thus the arrangement of tissue is such as to show in transverse section 

 sector-shaped masses of sclerenchyma united together and traversed by phloem and 

 xylem proper, some of the phloem-groups being situated at the ends of the radiating 

 arms of pith. In some sections, traversing the medulla there may be seen isolated 

 strands of xylem surrounded by sclerenchyma, and sometimes accompanied by strands 

 of phloem. 



Thus far the structure of ^r^m-roots is seen to agree with Mohl's description of Palm- 

 roots in general. At this point it may be noted that Mohl (' De Structura Palmarum,' 

 Engl, transl. p. 49) wrote : "the cells immediately surrounding the central body contain 

 on their inner side transverse fibrous thickenings like many anther cells," describing in 

 these words the tissue now classified under the name endodermis. 



A third type of section may be distinguished which differs from that just described 

 only in that the vascular mass or stele is not a smooth cylinder or cone, but a longitudi- 

 nally furrowed column ; conformably with this the endodermis does not appear as a 

 smooth circle in transverse section, but has a wavy outline ; it is, however, perfectly 

 continuous. 



The kind of structure referred to as type three has been described for the roots of 

 Iriartea. Mohl (loc. cit., Engl, transl. p. 50) wrote : " In the upper parts of many 

 palm-roots, e. g. Phcenix, Cocos, fibrous bundles are scattered through the rind, while no 

 trace of them is found in others. But the rather thick root of Iriartea exorhiza exhibits 



