Il8 TRANSPORT OF WATER AND SOIL SOLUTES 



Path of Water Ascent. — The path of the ascent of water 

 is clearly mapped out by placing a leafy stem in a weak solu- 

 tion of eosin or indigo-carmine. If the stem is then studied 

 by means of longitudinal and cross-sections before the stain 

 has had time to diffuse from the tissues through which it is 

 rising into surrounding tissues, it will be found that only the 

 tracheal tubes and tracheids are stained; and when the stain 

 has risen into the leaves it is the tracheids in the veins that first 

 show its presence. Furthermore, if the cut end of a stem having 

 foliage be dipped into melted gelatine or paraffin the tracheal 

 tubes will be plugged up for some distance, so that the end of the 

 stem can be trimmed to expose all the tissues without removing 

 the plugs from the tracheal elements. If the end of the stem 

 is then submerged in water the leaves soon wither, while in a 

 control experiment employing a similar branch with the tracheal 

 elements left open the leaves continue fresh and unwilted. These 

 and other experiments leading to similar results leave no ground 

 for doubt that the path of water ascent from the roots is through 

 the cavities of the tracheal tubes and tracheids. 



Influence of Environment on the Water-conducting 

 Tissues. — The amount of tissue devoted to the circulation of 

 water depends upon the intensity of the demand for water to 

 supply the loss by transpiration. In submerged water plants 

 where transpiration does not take place the tracheal elements 

 are hardly more than vestigial. In water plants with foliage 

 borne above the surface the water-conducting tissues are better 

 developed, while in land plants these tissues spring into still 

 greater prominence. The amplitude of water-conducting tis- 

 sues reaches its climax in tall-growing climbing plants with 

 slender stems and large crowns of fohage, where the distance 

 to be traveled is far and the demand for water through trans- 

 piration large; and in those trees also, such as the willows and 

 Liriodendron (yellow poplar^ of commerce) whose roots find 

 abundant water in the moist soil of intervales and along streams, 

 and whose foliage is lavish in transpiration (Fig. 6i). 



