186 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



The phloem (/) is small celled and not markedly distinct 

 from the peri cycle. 



The endodermis is followed by a fairly thick cortex (c) 

 limited by a piliferous layer (a) replaced later by an 

 exodermis (b). 



On examining the swollen roots one finds the number of 

 protoxylems greatly increased. A section of such a root as 

 that shown at figs. 2 or 3 may show from 12 to 20 pro- 

 toxylems united by a sclerotic medulla. 



The protoxylems stand out prominently, creating deep 

 bays in which the phloem lies (fig. 4). 



The pericycle is, in the swollen roots, much thicker, 

 averaging 4 — 5 cells in depth. 



The endodermis is again well marked and is followed 

 by a thick cortex. 



As the root becomes older two cambiums make their 

 appearance in a manner quite similar to that seen in a 

 normal terrestrial dicotyledonous root. 



The first beginning of the change is apparently the 

 development of a phellogen from the most external layer 

 of the pericycle. Tangential division in this layer results 

 in the formation of a distinct layer of phellem and is 

 followed by the shrivelling of the original cortex and 

 rupture and disintegration of it as well as of the endo- 

 dermis. At the same time, or more commonly somewhat 

 later, a cambium appears inside the phloem at the base of 

 each concavity formed by the prominent protoxylems, 

 becoming gradually completed from the pericycle out- 

 side the protoxylems. Secondary thickening, chiefly of 

 xylem, is now added in the ordinary way until the bays 

 become filled up with secondary xylem (figs. 5 and 6). 



In the flattened roots (fig. 7) as many as 50 protoxylems 

 or even more (fig. 8) may occur. 



