104 BEAUTIES OF BRITISH TBEES. t)EC., 



the beauties of the Privet are summed up in tlie freedom of its 

 branching in a most irregular bushy form, the persistence of its not 

 unpleasantly coloured foliage and its dense clusters of white blossoms. 

 Were they as hard, it cannot be denied that some of its Japanese 

 congeners exceed our European species in many points of beauty; but 

 we are not concerned here with any but native trees. 



G. S. BOUGLEE. 



THE SAP OF THE BIBCH. 



An interesting observation, says, a recent number of The Lancet, 

 of much importance in the cause of vegetable physiology, has been 

 communicated by Professor Attfield at a recent evening meeting of 

 the Pharmaceutical Society. A slender white Birch tree, with a stem 

 which at the ground is only 7 inches in diameter, having a height of 

 3'9 feet, and before any foliage had developed, would appear to be able 

 to draw from the gi'ound about seven-eighths of a gallon of fluid every 

 twenty-four hours. That, at all events, was the amount collected 

 from a branch with its transverse section exposed to the atmosphere. 

 The collected sap was a clear bright watery fluid, of a specific gravity 

 of about 1002*5, and containing not quite 1 per cent, of sugar. The 

 liquid also held in solution a ferment capable of converting starch 

 into sugar. When exposed for some time to the air it became charged 

 with bacteria, the sugar being replaced by alcohol. That the Birch 

 sap contains sugar is known to the peasants of many countries, 

 especially liussia ; and a kind of ' Birch wine ' is manufactured by 

 permitting the fermentation of the saccharine juice. The obscurity 

 which still surrounds the science of the circulation of fluids in plants 

 was rightly the subject of some remarks by Professor Attfield. In 

 the course of the discussion which ensued Professor Bentley said that 

 at this season of the year, before the leaves were expanded, the reserve 

 materials largely stored up in the roots were undergoing active 

 chemical changes ; this state was associated with an increasing density 

 of the fluids of the part, and the consequence was that an excessive 

 osmotic action took place. There was far more fluid absorbed 

 from the earth than the tree could use, and what botanists call ' root 

 pressure ' took place. This was a matter that had not been clearly 

 explained until the last few years ; it had nothing to do with the 

 leaves. The ' bleeding ' did not occur at any other time of the year, 

 for as soon as the leaves were fully developed, the fluid which was 

 absorbed by the roots was naturally carried up the tree and became 

 transpired, and thus dissipated. Probably the ascending sap had 

 greater force in summer, but the tree did not ' bleed ' when incised, 

 for the foliage carried off the fluid. We quite concur in the wish 

 expressed by Professor Attfield that if there had been time he should 

 have liked to have asked Professor Bentley to explain further what 

 * root pressure ' was. It seems at first sight to be a downright 

 petitio priiicipii ; but if, as Professor Bentley remarked, the density, 

 or, perhaps better, the tension of the contents of the roots were much 

 augmented as the result of intimate chemical changes, then a partial 

 explanation might be said to be afforded. 



