92 COMPRESSION. [CH. IV 



An interesting modification of the experiment is to 

 plunge the cut ends of transpiring branches into emulsion : 

 in this way the distribution of the transpiration current 

 in the cross section may be studied in various plants. 

 Diluted skim-milk stained black with osmic acid makes 

 a good emulsion. 



(106) Injection of vessels 1 . 



Cut two similar branches of Portugal laurel, Primus 

 lusitamca, place one in water, the other in melted cocoa- 

 butter (or gelatine), into which it must dip as deeply as the 

 vessel allows. After an hour, during which the cocoa- 

 butter is kept melted, take the branches out of the fluids, 

 and let them lie on the table till the cocoa-butter 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 2 . 



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

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

 wood must be chosen, young stems are too brittle. 

 Branches of bramble (Rubus fruticosus) also serve, but are 

 awkward to work with ; in winter, last summer's shoots of 



1 Elfving, Bot. Zeitung, 1882; Scheit, Bot. Zeitung, 1884; Errera, 

 Soc. R. de Bot. de Belgique, Vol. xxv. Part n. 1886. 



2 F. Darwin and E. Phillips, Cambridge Pliilosopli. Soc. 1883. 



