CONDUCTION OF FOOD. 



1. MECHANICS OF THE MOVEMENT OF THE EAW FOOD-SAP. 



Capillarity and root-pressure. — Transpiration. 



CAPILLARITY AND ROOT-PRESSURE. 



Unicellular plants make use individually of the food material which they 

 absorb from their surroundings, and work it up into the organic substances which 

 they require for their structure and increase in bulk, and also for the production of 

 future generations. In all plants composed, on the other hand, of aggregates of 

 cells, there is a division of labour. Of the protoplasts occupying the cell-cavities 

 of such larger plant-structures, one part provides for the absorption of the water 

 and food-salts, another for the taking in of the gases which are used as food, 

 and yet another part works up this food into organic substances for construc- 

 tive purposes. The centres in which these various industries are carried on are 

 frequently situated at some distance from one another, and it is obvious that 

 there must not only be some communication between the various regions of activity, 

 but that active forces must come into play which will effect the transport of the 

 food from the cells whose function it is to receive it, to those in which it is to be 

 elaborated into building material. It is evident that the greater the distance is 

 between the various centres of the plant in question, the more difficult will be the 

 performance of this task. In aquatic plants and lithophytes, all of whose superficial 

 cells have the power of taking in nourishment from their environment, these 

 distances are proportionately small, while they attain their greatest dimensions in 

 land-plants whose roots are embedded in the earth, and whose leaves are surrounded 

 by air. In trees the food materials which are taken up by the absorbing roots 

 beneath the ground must frequently travel far more than 100 metres before reach- 

 ing the topmost leaves. The path to the summit is very steep, and the fluid in 

 rising must be able to overcome the force of gravitation, which has no inconsider- 

 able significance at heights such as these. 



Naturally, desire for knowledge has at all times directed attention to this 

 phenomenon, and the most diverse attempts have been made to explain by what 

 means the food-sap taken in by the roots of trees is enabled to reach their 

 summits. It was first considered to be in virtue of capillarity; that just as oil, 

 alcohol, or water, is drawn up the wick of a lamp, the liquid food can rise in the 

 delicate tubular cell-formations called vessels, which, united together in groups or 



