376 The Physiology of Plants BOOK in 



remembered that ten years earlier cane sugar and the pro- 

 ducts of its inversion had been discovered in foliage leaves 

 by Kayser. The experiments of Brown and Morris led 

 them, as we have said in a former chapter, to the view 

 that cane sugar is the first-formed sugar of the leaf. Their 

 explanation of the contents of the translocation stream is 

 that the maltose arises from the starch, and the glucose 

 and fructose come from the inversion of cane sugar. They 

 found evidence of the existence in leaves of the enzyme 

 invertase, which effects this change, and which had been 

 known since 1860 to be a factor in the metabolism of 

 yeast. 



The transport of proteins has not been the subject of 

 such prolonged inquiry. In 1872 Pfeffer ascertained that 

 the results of their transformation as detected in the 

 travelling stream take the form of some of the amido- or 

 amino-compounds, which we have seen to be probable 

 stages in their formation. An enzyme capable of setting 

 up the decomposition which leads to the production of such 

 compounds was first investigated by Wiirtz in 1879. He 

 found it originally in the fruit of the Papau-tree, but later 

 prepared it also from the sap of the stem and leaves. 



The occurrence of proteoclastic enzymes in the leaves 

 remained somewhat doubtful till the end of the century, 

 when the researches of Vines established it satisfactorily 

 and gave us for the first time anything like a clear idea 

 of the nature of the processes involved. To this point we 

 shall return later. 



Turning to consider other features of the translocation 

 process we may consider the localization of the paths of 

 transport. The general idea of a stream of sap circulating 

 continually round the plant did not long survive inquiry, 

 but nothing replaced it very definitely for some time. 

 Sachs held the path of sugar and of whatever might be 

 the travelling form of combined nitrogen to be the paren- 



