84 COMPRESSION. [CH. IV 



branches out of the fluids, and let them lie on the table 

 till the cocoa-fat is quite cold : cut fresh surfaces to both 

 and place them in watery solution of eosin. After an 

 hour or two the progress of the eosin may be compared 

 by cutting off shavings of bark at various heights in the 

 two branches. 



(107) Compression. 



To prove that the transpiration current travels in the 

 vascular cavities, it may be shown that squeezing the 

 tissues in a vice checks the upward stream of water'. 



Cut, under water, a leafy stem of Helianthus tuherosus, 

 and fit it to Kohl's apparatus. A plant with well formed 

 wood must be chosen, — young stems are too brittle. 

 Branches of bramble also serve, but are awkward to work 

 with ; in winter, last summer's shoots of ivy answer fairly 

 well. Do not attempt to compress the whole stem but 

 cut away half of it before applying the vice. The exposed 

 surface may for greater security be rubbed with lard to 

 prevent air leaking into the vessels exposed ; in any case 

 the part selected for compression must be as far as 

 convenient from the cut end, so as to avoid the chance of 

 air being sucked back into the apparatus. The vice 

 should be a light one so that it may support itself when 

 it is screwed on to the stem. 



When the rate of absorption is steady, compression 

 may be applied : it will be found necessary to screw the 

 vice with great force so that the compressed tissues are 

 squeezed to a mere plate. If the compression has been 



1 F. Darwin and E. Phillips, Cambridge Philosoph. Soc. 1886. 



