AEEIAL BOOTS OF HEDEBA HELIX. 187 



Similar instances of secondary thickening in aerial roots 

 are known to occur in certain Melastomacese, but I am 

 not aware that the phenomenon has been previously 

 observed in Hedera ; at all events I have been unable to 

 find any reference to the subject in the literature. 



Explanation of Plate. 



Fig. 1. Transverse section of normal aerial root of 



Hedera helix. 

 Figs. 2, 3 and 7. Abnormal roots of Hedera. 

 Fig. 4. Transverse section of abnormal root, showing a 



large number of prominent protoxylems. 

 Fig. 5 and 6. Development of phellogen and cambium 



in abnormal roots. 

 Fig. 8. Transverse section of abnormal loot with 50 



protoxylems. 



