60 TRANSPIRATION AND ASCENT OF SAP ch. 



the walls of the trachea?, and by the cells adjoining the 

 water tracts above the heated region. In the leaves, those 

 which are not in a form suitable for assimilation must 

 accumulate ; and, if sufficient of the branch has been 

 killed, the accumulation will ultimately without any 

 other poisonous action plasmolyse the cells of the leaf. 1 



Reduction in the water-supply may be also brought 

 about by the coagulation of colloids in the sap, and the 

 consequent formation of plugs in the conducting tubes. 

 This condition, as has been noted above, has been observed 

 by several investigators. 



Poisonous action of contamination. It seemed 

 of interest to essay to find out if the sap in steamed 

 branches contained any substance which acted as a 

 protoplasmic poison, and not merely as a plasmolysing 

 agent by simple accumulation. To test this point, saps 

 extracted from branches subjected to various treatments 

 were applied to severed leaves of Elodea canadensis, and 

 the effect on the cells of these leaves microscopically 

 controlled. 



In the first place it was found that the cells remained 

 normal, and protoplasmic streaming continued undimin- 

 ished in the sap from fresh branches for at least five days, 

 and probably much longer. This point was verified in 

 the case of the sap of Ilex aquifolkim, Prunus cerasus, 

 Syringa vulgaris, Cotoneaster frigida, and Salix babylonica. 

 In contrast to the sap from the fresh branches, that from 

 the steamed branches of all these, with the exception of 

 Ilex aquifolium produced lethal changes in the leaves of 

 Elodea within two or three days. These changes consisted 

 in a cessation of protoplasmic streaming, in the discolora- 

 tion of the veins and margins, and in the contraction of 

 the protoplasts of the cells all over the leaf, and their 



1 It may be noted that Ursprung looked for the production of plasmolysing 

 effects in the root-hairs of Impatiens svltani by a decoction of the same plant, 

 but did not tind any. Here, of course, concentration would not take place. 



