202 BOTANY part i 



mattei", asparagiii, acids, and especially carbohydrates). The amount 

 of saccharine matter in the sap of some plants is so great that sugar 

 may be profitably derived from it. The sap of the North American 

 sugar maple, for example, contains h per cent of sugar, and a single 

 tree will yield 2-3 kilos. The sap of certain plants is also fermented 

 and used as an intoxicating drink (birch wine, palm wine, pulque, 

 a Mexican beverage made from the sap of Agave, etc.). One 

 inflorescence of Agave will yield 1000 litres of sap in from four to 

 five months. 



Causes of the Excretion of Water (^-). — The excretion of drops of water from the 

 intact plants is in part due to an active excretion of water from superficial cells. 

 In other cases water is forced into the vessels, and finds a way out at the points 

 of least resistance. In the phenomenon of bleeding, also, water is forced from 

 parenchymatous cells into the cavities of the vessels ; although this process takes 

 place especially often in roots, it is not always absent in the cases of stems and 

 leaves. Water may also be excreted into the cavities of intercellular spaces, e.g. 

 in the hollow internodes and leaf-stalks of Ciicurbita ; in this case, on need arising, 

 the water may again be absorbed. The phenomenon of root-pressure may be 

 determined or increased by the stimulus of wounding or by the processes of healing, 

 as well as by an excess of water. This was shown by Molisch for the inflorescences 

 of Palms, and in borings made in our native trees. The amount of fluid excreted 

 from the callus tissue in the latter case was small, but was forced out even under 

 a high pressure (up to 9 atmospheres) Q'^). 



Thus, when fully analysed, all the phenomena described show a one-sided 

 excretion of fluid from living cells. That this does not always result from the 

 same cause is indicated by what was stated above, since the fluid is sometimes 

 nearly pure water, at other times more or less concentrated sap. When the fluid 

 contains an appreciable amount of dissolved substances, the protoplasm at the 

 place of its excretion must have been permeable to these substances. At this 

 point of the cell the same osmotic pressure cannot have been produced as at a spot 

 when the protoplasm was impermeable. The result is a one-sided extrusion of 

 fluid, which may take place with a force as great as the diS'erence between the 

 osmotic pressures on the side with impermeable and that with permeable proto- 

 plasm. When the excretion is more highly concentrated, as for instance in 

 nectaries, other factors come into play. The substances outside the cell (sugar), 

 whether they have been excreted from the cell or have arisen in the cell-wall, must 

 exert an osmotic action and withdraw water from the cell. When, however, as in 

 the Vine, the excreted sap is almost or quite pure water, its extrusion only becomes 

 possible by the maintenance of a dift'erence of concentration of the cell-saj) at 

 the dilferent sides of the cell. 



Conduction of Water (^*) 



The water, which is partly given oiF in the form of vapour, 

 especially from the leaves, and in part exudes in the fluid form from 

 hydathodes and wounds, has, as a rule, been absorbed by the roots. 

 It has thus to traverse a path which, even in annual plants, may 

 amount to some metres, and in the giants of the vegetable kingdom 

 may be much more than 100 m. ; the stems of Eucalyptus amygdalina 



