185 



On SECONDAEY THICKENING in the AEEIAL 

 BOOTS of HEDEEA HELIX. 



BY 



E. J. Harvey Gibson, M.A., F.L.S. 



Professor of Botany in University College, Liverpool. 



With Plate III. 



[Read January 13th, 1899.] 



I eeceived lately, through the kindness of Miss May 

 Eathbone of Neston, Cheshire, a portion of an ivy plant 

 whose aerial roots appeared to have undergone a some- 

 what curious change. Instead of showing the usual small 

 aerial roots of from half-an-inch to an inch in length, the 

 branch was provided with roots 6 or 8 inches long, pro- 

 fusely branched and considerably swollen especially for 

 the last 2 inches of their length. 



The plant had been grown in a large pot in a room and 

 trained up the side of the window, and from about a foot 

 above the pot up to the topmost branches which twined 

 round the curtain rod exhibited both normal and abnormal 

 roots. They developed from the stems at all angles, quite 

 irrespective of light, and often all round the stem. 



The normal aerial root of ivy shows, as has been 

 described and figured by Van Tieghem (L'origine des 

 membres endogenes, 1889) and others, a central vascular 

 cylinder surrounded by a well marked endodermis (d) 

 fig. 1. There are, as a rule, five protoxylems (h) united 

 centrally by a strongly sclerotic medullary region (g). 

 The pericycle (e) is, as a rule, from one to two layers thick 

 between the phloem and the endodermis, but may be three 

 layered opposite the protoxylems. 



