184 



BOTANY 



their ccmrse and direction is thus obtained. By placing the roots upon 

 litmus paper, it may be demonstrated that the corrosion is due to the 

 action of an acid. 



The nutrient water with which the cell walls of the epidermal cells 

 and root-hairs first become permeated is taken up by the epidermal 

 cells, and thence passes through the cortical cells and the endodermis 

 (p. 113) to the central cylinder of the root. 



The Distribution of the Nutrient Water — 1. Eoot-Peessure. — 

 The causes which determine the direction and strength of the movement 

 of the water through the living cells of the root-cortex into the vascular 

 bundles are not yet fully understood. The fact that the water does 

 actually pass into them, but at times indeed is forced into them with 

 a considerable pressure, may be easily demonstrated. If the stem of a 



strongly-growing plant, such as the Sun- 

 flower, Dahlia, or Indian Corn, be cut off 

 close above the ground, and the cut 

 surface dried and then examined with a 

 magnifying-glass, water will, in a short 

 time, be seen to exude from the severed 

 ends of the bundles. By close inspection, 

 it is also possible to determine that the 

 water escapes solely through the vascular 

 or woody portion of the bundles. When 

 the soil is kept warm and moist the out- 

 flow will be greater, and will often con- 

 tinue for several days. During this time, 

 a half-litre or more of water will be dis- 

 charged. This water, as analysis shows, 

 is not pure, but leaves on evaporation 

 a residue of inorganic and organic sub- 

 stances. 



Again, if a hollow glass tube be placed 

 on the root-stump and tightly fastened by 

 rubber tubing, the exuded fluid will be 

 forced up the glass tube to a considerable 

 height. How great the force of this 



Fig. 174.— Vigorous exudation of water ° ° 



as the result of root-pressure from pressure is may be shown by attaching 



an S tube to the stump and closing it 

 with mercury (Fig. 174). The column 

 of mercury will in some cases be forced 

 to a height of 50 or 60, and under favour- 

 able conditions to 100 or more centi- 

 metres, thus indicating a root-pressure 

 which may sometimes considerably exceed 



one atmosphere, and is of sufficient power to raise a column of water 



6, 8, and 1 3 metres high. 



a cut stem of Dahlia variabilis. 

 The smoothly cut stem s is joined 

 to the glass tube g by means of 

 the rubber tubing c. The water 

 W, absorbed by the roots from the 

 soil, is pumped out of the vessels 

 of the stem with a force sufficient 

 to overcome the resistance of the 

 column of mercury Q. 



