528 dufrenoy: false witches'-brooms in Ericaceae 



ascent of water, resulting in partial starvation of leaves, a proc- 

 ess which these in turn may resist by means of xerophilous 

 adaptations. 



In Ericaceous plants it should be noted however that: 



(1). The vessels of the xylem are never obstructed by mycelia, 

 either in brooms of Arbutus infected by Exobasidium unedonis 

 Maire + Gloeosporium conviva Maire, or in brooms of Vaccinium 

 myrtillus infected by Gloeosporium vaccimi sp. nov. On the 

 contrary the author found the vessels of the xylem to be notice- 

 ably wider in infected leaves of Vaccinium than in sound ones. 

 A reduction of transpiration of brooms, if recorded, should not 

 therefore be ascribed to interference with water ascent. 



(2). Infected leaves are rolled up, affording less surface to 

 transpiration. 



(3). The infected leaves are often colored red by anthocyan, 

 and such a coloration might be considered to cause a modifica- 

 tion of transpiration. 



The results from experimental work by ' the author were as 

 follows : 



Leaves of false witches'-brooms of Arbutus show greater 

 transpiratory activity than neighboring normal leaves. This is 

 due to the fact that the infected leaves remain ''juvenile" while 

 sound leaves mature and become thickly cutinized. 



Transpiration, as directly measured by the chloride-cobalt 

 paper, in the sun and in the shade, is shown to be ten times 

 more active for leaves of Vaccinium myrtillus and Arbutus 

 unedo infected by Gloeosporium, than it is for sound leaves, and 

 this quite irrespective of pathological reddening by anthocyan. 

 Cut-off false witches'-brooms may remain living for a week 

 when entirely immersed in water. If, however, the base of the 

 twig only is watered, absorption cannot offset the excessive 

 transpiration of the leaves, which wither and dry up within an 

 hour of exposure to the sun. Transpiration then becomes so 

 restricted that a very small loss of water is indicated by weigh- 

 ing cut twigs during several days, and xerophytic adaptation 

 might falsely be concluded to exist. 



