[Chap. XXII THE MOVEMENT OF MATERIALS IN PLANTS 197 



ing for all the intricate physico-chemical conditions of which it is a 

 consequence. 



For many years students of plants have been trying to explain the 

 movement of materials into, out of, and within plants. Not all the ques- 

 tions raised have been satisfactorily answered, but enough has been 

 learned for botanists to conclude that these movements are dependent 

 upon the same physical processes that occur in non-living systems. Most 

 of them have already been imitated in experiments by means of labora- 

 tory apparatus. These movements will now be considered in more detail 

 in relation to some of the physical processes involved. 



Mass movements. The flow of water in a stream and the movement of 

 a current of air (wind) along the street are familiar examples of what 

 is meant by mass movement. Frequenth' it is a flowing movement or 

 current. Mass movements of materials may occur in plants. If you put 

 your finger in water and then withdraw it, a film of water adheres to it 

 because of the cohesion of the molecules of water, and the adhesion 

 of the molecules of the water and those of the skin. These cohesive and 

 adhesive forces between molecules at the surfaces of objects may result 

 in tensions, called surface tension. Surface tension may cause the move- 

 ment of water for short distances, and any substances dissolved or dis- 

 persed therein move with the water. For instance, if two microscope 

 slides are held closely together and one end is placed in a shallow dish 

 containing a dye dissolved in water, a small mass of the solution quickly 

 moves up between the two slides. This is a familiar phenomenon to 

 anyone who has frequently washed slides or dishes. Similarly, water 

 may move into the small spaces between the fibers of a blotter, a piece 

 of wood, or a plant, or into the pores of the soil. This movement of 

 water into small spaces as a result of surface tension is often referred 

 to as capillarity. 



We have already seen that protoplasm may move about within a cell 

 by flowing movements (protoplasmic streaming). Substances, such as 

 food and salts, that are in the protoplasm are of course carried along 

 in the same manner that a log is carried in a stream. There may even 

 be some movement of materials in this way between cells when strands of 

 protoplasm extend from cell to cell through minute pores in the cell 

 walls. Surface tension is one of the forces which cause protoplasmic 

 streaming. 



There is also a streaming, or mass movement, of water up the con- 



