INFLUENCE OF WATER ON PLANT-LIFE 25 



and if this condition is prolonged the plant dries up 

 and dies. 



4. The materials derived from the soil and absorbed 

 by the roots are conveyed in a current of water which 

 passes up the stem to the leaves. This ascending stream 

 of sap is called the transpiration-current, and the main- 

 tenance of its flow is necessary to the healthy life of the 

 plant, for, like the stream of blood in animal bodies, it 

 conveys a cargo of materials by the utilization of which 

 the plant is enabled to live. The 

 path of the water is through the 

 woody tissues which constitute the 

 greater part of the vascular system 



d. 



FIG. 2. LONGITUDINAL SECTION OF PART or A 



ROOT-TIP, SHOWING OUTER TISSUES or ROOT 

 (a) AND ROOT-HAIRS (6), SURROUNDED BY 

 SOIL-PARTICLES (c). (HIGHLY MAGNIFIED.) 



FIG. l. SEEDLING OF in roots and stems. From the stems 



MUSTARD, SHOWING grea t trunk-veins are given off to 



(NATURAL Siz R E S ) *be leaves, where they break up 



into a network of capillaries. Here 



the water is yielded up to the living cells, and with it 



the nutrient material contained in it. 



Root- Absorption. Most of the higher plants live rooted 

 in the ground. Water enters the plant by the roots. 

 The actual entrance is effected through the root-hairs 

 tiny, hair-like cells which are found clothing the roots 

 near their tips (Figs. 1 and 2). The water passes through 

 the walls of these root-hairs by a physical process known 

 as liquid-diffusion, or osmosis (see p. 91). 



