522 ORGANOLOGY. 



3. The principle of the activity of the sap-stream in plants is principally 

 found in evaporation, and perhaps, in a slight degree, in the chemical 

 processes, through which a voluminous fluid is reduced to a smaller 

 quantity of solid matter. 



4. Evaporation and the chemical process often determine the direction 

 of the sap-stream. New fluid is drawn up only into those parts where the 

 already existing fluids evaporate, or are subject to chemical changes. 



5. There is no reason for supposing that there is a returning stream of 

 sap, since cells that are already full can receive no more. 



6. A stream of sap passes from the absorbing cells to those where the 

 greatest chemical activity and the greatest evaporation is going on, and 

 both of these are found united together in the youngest and extreme points 

 of most Phanerogamia. 



7. Annual plants wither from below upwards ; perennial plants of 

 our climate, in the chemical inactivity of winter, die also first from below ; 

 the motion of the sap of both, or, at least, of the active stream of the 

 summer period, is terminated in such a manner, that the surplus sap 

 retires into the youngest and extreme points, and from thence escapes. 

 Annual plants carry naturally all their soluble matter into these external 

 evaporating parts ; consequently, a cultivated land will exhaust the soil 

 more when the herbage is cut ripe, or nearly so, than when cut green, 

 because in the latter case more than half the substances remain on the 

 fields with the stubble. Not only have the ripe plants taken up from the 

 soil double as much in their long period of vegetation, but the most im- 

 portant matters, alkalies and soluble phosphates, are not equally distributed 

 in the plant, but are accumulated in those parts carried away at the 

 harvest. 



Each cell now assimilates the sap, which is a longer or shorter time 

 entering, according to the nature of the cells, that is to say, according to 

 the chemical process, which is regulated by their first origin ; and each 

 must give out again as much of its contents as it has taken up by endos- 

 mosis from other cells. The absorbed fluid is distributed through the 

 whole plant as it is required, that is to say, according to the demands of 

 the individual chemical processes. As water is continually exhaled by 

 plants in proportion to the dryness, motion, and warmth of the air, so the 

 sap becomes concentrated, and thus interrupts the endosmotic process 

 towards the other cells ; this action is continued naturally downwards 

 towards the root, by which new watery and unassimilated fluids are 

 absorbed. If this stream of crude sap is artificially interrupted in its 

 course from below upwards, the sap in the upper part becomes more 

 concentrated, and its organising power increased. This is the simple 

 fact which lies at the foundation of all those phenomena which are brought 

 forward to support the groundless hypothesis of a descending bark -sap. 

 The two most important facts upon this subject are, 1. The magic ring 

 (ringing fruit-trees), 2. The action of grafts. If from the circumference 

 of a branch or tree a ring of bark be removed, the upper part will bear 

 richer blossoms and fruit ; the latter will ripen quicker, the leaves will be 

 thrown off sooner, and the trunk will become thicker and stronger than in 

 the part below the cutting. All this is completely explained by the fore- 

 going facts, without making it in the least degree necessary to assume the 

 motion of any descending proper juice or bark-sap, which certainly does 

 not exist.* When an Apricot-graft grows from the trunk of a Plum-tree, 



* The effects of ringing the bark remains the same if the branch be bent down, but 

 not if it be turned back, as the ascending sap immediately enters ; if the upper, instead 



