in ASCENT OF SAP IN STEMS 57 



A priori sucli a result seems inevitable. When the heat 

 has killed a portion of the stem, the cells adjoining the 

 water-tracts become permeable ; and hence the dissolved 

 substances in their vacuoles are set free into the upward 

 current of sap in the tracheae. The vacuoles contain acids, 

 carbohydrates, and salts, so that, even in the absence of 

 corroborative observations, we should expect the sap to be 

 enriched with these substances Furthermore, very pro- 

 bably substances in the cells ordinarily not in solution 

 would be brought into solution, and introduced into the 

 sap by the higher temperature ; possibly, too, some bodies 

 might be precipitated from the rising sap by the higher 

 temperature. Yet another change is to be anticipated. 

 The heat will destroy any thermolabile substances in the 

 sap and in the adjoining cells. Coagulation changes may 

 also be expected. 



It is not difficult to test these surmises experimentally ; 

 and, indeed, a colour-change in the sap issuing from heated 

 stems has before now been recorded and commented upon. 



The sap extracted from various trees by means of a 

 centrifuge provided material suitable for this investiga- 

 tion. Short lengths of the branch to be investigated 

 (9-10 cm. long by 2-2'5 cm. diameter) are placed in gilt 

 buckets of a centrifuge, and the sap yielded after about 



those which wilt owing to a lack of water. In the former case the margin 

 of the leaf first becomes darkened and this darkened region gradually invades 

 the leaf between the veins. It then dries and shrivels while the green 

 parts immediately round the veins remain comparatively fresh. As this 

 change is taking place these veins usually become pink and finally brown. 

 This coloration is particularly noticeable when the leaves are viewed with 

 transmitted light. Shrivelling and withering of the leaf, except at the 

 edges, does not occur until after these changes are complete. 



On the other hand, when leaves fade simply from an insufficient water 

 supply, e.g., on a branch severed from a tree, shrivelling comes on while 

 they are still green. Blackening appears only after shrivelling and occurs in 

 irregular patches. The veins do not change colour and the walls of the 

 trachea? do not appear coloured in transverse section. The first colour 

 change is when the cell-contents of the mesophyll and parenchyma of the 

 veins colour brown after death. 



