PITH-EAY FLECKS IN WOOD. 9 



long, as in river bircli, the wound tissue consists of cells with corres- 

 ]:)ondingly larger cavities. Division in this case takes place more 

 tardily. Even under the stress of wound stimulus, the inherent char- 

 acter of cell formation persists. There seems, then, to be a definite 

 relation between the ray cells and the cells of the wound tissue. 

 The relation, however, of the vessels to wound tissue is more obscure. 

 In woods with small vessels there is usually a corresponding reduc- 

 tion in the cells of the wound tissue; while in woods with large vessels 

 the reverse is true. Though no logical reason can be given for such 

 a relation, present observations confirm it. 



When viewed in tangential (bastard) section, the passages appear 

 as broad bands of wound tissue. All appearance of pith-ray forma- 

 tion is lost. The cells are commonly isodiametric and polygonal in 

 form (PI. V, fig. 2), resembling in some respects the cells of the pith or 

 medulla. A radial view (PI. V, fig. 1) presents no new characters. 

 The wound tissue is not as extensive in this section, however, since 

 the larva works only in the cambium. 



GROUPS AND SPECIES OF TREES AFFECTED. 



Pith flecks are present in a greater number of tree species than is 

 generally supposed. Nordlinger reported them as occurring in 41 

 genera, included in 27 families. A critical examination of sections 

 prepared by him increased the list of hosts still further. It is possible 

 that the markings found are all the work of the larvae of a dipterous 

 insect, but only extensive observations can determine this. That 

 they are all caused by cambium miners is, however, apparent. No 

 other injury would heal in the manner described. 



As far as determined by the present investigation, pith flecks occur 

 in only 5 families of trees in the United States, namely, the Sali- 

 cacese, Betulacese, Rosacese, Aceracese, and Tiliacese. The followhig 

 is a tentative list of woods indigenous to the United States in which 

 pith flecks were observed : 



SALICACEiE. 



Common name. Botanical name. 



Bebb willow Salix bebbiana Sargent. 



Long leaf willow Salix fluviatilis Nutt. 



Hooker willow Salix hookeriana Baratt. 



Smooth leaf willow Salix laevigata Bebb. 



Black willow Salix nigra Marsh. 



Nuttal willow Salix nuttallii Sargent. 



Silky willow Salix sitchensis Sans. 



Narrowleaf cottonwood Fopuhis angusti/olia James. 



Balm of Gilead Populus balsamifera Linn. 



Cottonwood - Populus deltoides Marsh. 



Largetooth aspen Populus grandidcntata Michx. 



Aspen Populus tremuloides Michx. 



Black cottonwood Populus trichocorpa Torr. tt Gr. 



