260 PLANT PHYSIOLOGY 



being diluted by incoming water. In this region the endodermal 

 cells are also especially compact, with no intercellular spaces. 

 These structural characteristics should be kept in mind during 

 the following discussion. 



That the water rises in the wood can be shown by two separate 

 lines of experimentation. First, when a tree is girdled, i. e., when 

 a strip of the bark or cortex is removed from around a tree, the 

 leaves do not wilt, and the tree remains active for many days or 

 even weeks. If, on the other hand, by a very careful operation 

 the wood is removed for only a very short distance while the cortex 

 is allowed to remain, the branch will show signs of wilting within 

 a very few minutes if placed in dry air. This shows that the wood 

 is the channel of water ascent, but does not tell us whether the 

 water passes through the lumina (openings) of the wood cells or 

 through their walls. The former would naturally be supposed to 

 be the case, but so great a botanist as Sachs held that the water 

 traversed the walls rather than the lumina. That he was wrong 

 can be demonstrated by plugging up the lumina with wax, par- 

 affin, or gelatin, in which case the plant soon wilts. Furthermore, 

 plants with very large vessels (some of the climbers), which use 

 up large quantities of water, can be placed in water to which 

 drawing ink containing finely divided carbon has been added. 

 In this case, the water can be seen moving into the open ends of 

 the ducts, its path being rendered visible by the opaque carbon 

 particles it carries. Experiments with water containing dyes such 

 as eosin, lead to the same conclusion. 



On the whole, then, we can be certain that the transpiration 

 stream rises through the lumina of the wood cells. Inasmuch as 

 the cell walls imbibe water, some water may also rise in them, 

 and it is even possible that some goes up in the cortex; but the 

 water which rises in these places would never keep a plant from 

 wilting even though the roots were abundantly supplied. 



Role of Osmosis. — The principles of osmosis permit us to 

 understand how the water passes from the root hair to the central 

 cylinder and from the parenchyma cells surrounding the leaf 

 veins to the cells from which the water ultimately evaporates. 

 In the former case, it can be seen that an osmotic gradient be- 

 tween the root hair and the central cylinder will exist which will 

 favor the entrance of water and its osmotic movement towards 

 the central cylinder. As water comes into the root hair the osmotic 



