STEMS AND THEIR FUNCTIONS 163 



transpiration (evaporation) from the cells surrounding the air spaces of 

 the leaf. The suction force of these cells consequently rises, and they take 

 water from the adjoining cells, which in turn take water from the tracheids 

 in the fine vascular bundles (veins) of the leaf. The water columns in 

 these tracheids are continuous with those in the rest of the xylem, so 

 that withdrawal of water from the leaf tracheids creates a tension that is 

 transmitted throughout the plant and thus to the roots. In the roots the 

 transpiration pull is felt by the cells adjoining the xylem strands, and 

 water is drawn from them into the xylem tubes. This increases the suc- 

 tion force of the inner cortical cells and, combined with the osmotic gra- 

 dient already mentioned, leads to movement of water across the cortex 

 and intake of soil water by the epidermal cells and root hairs. 



There are certain difficulties involved in accepting this as the whole 

 story of water intake and upward transport in the plant. The develop- 

 ment of air bubbles in any great number of the xylem conduits would 

 render those channels useless, seriously diminish the flow of water, and 

 cause wilting or death of the plant. How this is prevented or overcome is 

 not understood. Again, water filaments under tension in glass tubes are 

 broken by a slight jar, while a tree can thresh wildly in the wind without 

 breaking them. Perhaps the greater resiliency of cellulose tubes as com- 

 pared with glass accounts for this. In spite of such difficulties transpira- 

 tion pull is the most satisfactory explanation yet found for the rise of 

 water in plants. The water columns, established by capillarity or root 

 pressure while the plant is small, are continuously maintained and pulled 

 upward during the growth of the plant, like so many slender wires hanging 

 suspended from the leaf surfaces. In this way they can be lifted hundreds 

 of feet into the air. The reality of transpiration pull becomes evident 

 when one cuts into the base of a tree; air enters the cut ends of the tubes 

 and follows the unsupported water columns as they are drawn upward. 



Transport of solutes. Prior to 1920, botanists were generally agreed 

 that upward transport of solutes, both organic and inorganic, occurs in 

 the water-conducting conduits of the xylem and that downward transport 

 takes place chiefly in the phloem. In that year two papers appeared, one 

 by Curtis, giving the results of experiments that seemed to prove that 

 both upward and downward transport of sugars takes place in the phloem, 

 and one by Birch-Hirschfeld, indicating that, on the contrary, both 

 upward and downward transport takes place through the xylem. The 

 resulting controversy led to much further experimentation, and while the 

 results are not altogether consistent, the problem has been somewhat 

 clarified though not simplified. 



It now appears that while the prime function of the xylem is to act 

 as a one-way transport system for water, it may under some conditions 

 and in some plants also carry inorganic salts taken in by roots and also 



