Dec. 9, i9i8 Seedling Diseases of Conifers 553 



figure 7 could not be readily produced by bending red pine, as the stems 

 broke squarely off, and immediately wilted. In one or two cases, however, 

 breaking over with one-sided lesions was obtained without apparent 

 rupture of the epidermis. In western yellow pine repeated bending, not 

 carried quite far enough to break the stem, in most cases produced a 

 collapse of the cortex at a point on the side away from the application of 

 the bending force, with a resultant weakening of the stem at this point 

 so that a sharp right-angled bend could be made without further injury 

 to the stem. In long repeated bending the entire perimeter became more 

 or less constricted at this point and the seedling lost its ability to stand 

 erect. With jack pine, the species in which the supposed wind injury 

 was observed in the seed beds, similar lesions were more easily produced 

 either by artificial bending or by the application of a strong draft of air 

 from a pressure vent. If the mechanical bending included a slight twist- 

 ing movement or if the seedling was so placed that it was whipped side- 

 ways as well as dov/nward by the blast of air directed against it, the col- 

 lapse of the cortex at the point of bending could be made even more 

 complete than that observed in the nurseries. The whitening of the stem 

 both above and below the constriction, shown in Plate B, figure 7, was 

 not produced by artificial bending or exposure to air currents to any 

 great extent except in lesions which occurred just at the soil surface. 

 It is believed that the change of the epidermis above and below the point 

 of constriction from red to nearly white, indicating the loss of sap from the 

 cells, came as a result of bruising against the soil rather than as a result of 

 bending alone. The stems of young seedlings are so susceptible to bruising 

 that even stroking the side of the stem with a smooth piece of wood or a 

 rubber-tipped rod results in the development of a lesion characterized 

 by sunken surface and more or less discoloration. 



What apparently takes place in these cases is the loss of sap from the 

 cortical tissue, and probably, in view of the loss of red pigment, from the 

 epidermal cells as well. Whether this is a result of an actual rupture of 

 the internal tissues or from an increase in the permeability of the tissue 

 as a result of the mechanical stresses developed is not known. It should 

 be pointed out that there is nothing especially different from the collapse 

 of the cortex under bending and collapse due to external pressure. In 

 both cases the injury is believed to be caused by pressure. This belief is 

 supported by the fact that in the case of experimental bending it is the 

 cells on the side of the stem on the inside of the bend which first collapse, 

 these, of course, being the ones which get the most pressure in the course 

 of the bending. That the vascular tissues are not seriously affected is 

 evidenced by the continued turgor of the leaves. In a seedling with a 

 much more serious and extensive lesion than the one figured, resulting 

 from mechanical bending, the leaves were still turgid 16 days after the 

 injury, and the prostrate stem had shown negative geotropic response, 

 88094°— 19 3 



