Chapter IX 

 UPTAKE AND MOVEMENT OF WATER IN PLANTS 



Introduction : — The simple lower plants, having no specialized tissue 

 for absorption and movement of v^^ater, are either limited in their habitat 

 to a moist environment or they remain small in size and are adapted to 

 periodic desiccation. Our common higher plants have developed specialized 

 tissues and organs for the rapid uptake and transport of water ; by means 

 of these, and special protective layers that greatly restrict water loss, they 

 are able to survive with a large portion of their surface exposed to the rel- 

 atively dry environment whilst their living cells enjoy virtually an aqueous 

 medium. In such plants there is a continuous but varying competition for 

 water between the plant as a whole and the environment, and the struggle 

 for water throughout the life of the plant is not confined to living cells but 

 exists among dead cells and their aqueous contents, cell walls, vacuoles, 

 protoplasm, and air spaces. When the absorption rate is high and transpi- 

 ration low, competition is at a minimum. When uptake is reduced or water 

 loss increased, competition for water increases and reaches a maximum dur- 

 ing advanced stages of wilting if the water supply to the roots fails. Periods 

 of stress occur diurnally in most plants due to excess transpiration during 

 midday ; seasonal trends are noted as annual plants pass through stages of 

 maximum growth and maturation; annual trends take place in perennial 

 plants during similar growth stages. Seedlings growing in moist soils and 

 a moist atmosphere show low stress ; in fact, they often have excess pres- 

 sure in their xylem conductors as evidenced by the forcing of liquid drops 

 out of hydathodes (guttation) or by exuding liquid from the xylem when 

 excised. Plants in water culture may exhibit guttation even when fully 

 grown, particularly during the night and early morning hours; stress in 

 the xylem of such plants may be demonstrated during midday by the rapid 

 inrush of dye when the stems are cut under a dye solution. Plants growing 

 in soil will guttate only if the soil is maintained near its field capacity and 

 atmospheric humidity is high; as soil moisture is depleted stress mounts 

 in the xylem, and throughout the plant. When the moisture content of the 

 soil reaches the range known as the permanent wilting percentage, the plant 

 is nc longer able to absorb appreciable amounts of moisture ; if more is not 

 supplied it eventually dies. 



Water Absorption: — Most plant physiology texts {cf. Miller, 1938; 

 Meyer and Anderson, 1939) describe the structure of roots and explain 

 the relation of gross structure to the function of water absorption. Kramer 

 has reviewed the literature on soil moisture in relation to plant growth 



(1944) and on the absorption of water by plants (1945). M agist ad 



(1945) has covered the relations of salinity and alkalinity to plant growth. 

 Water absorption will receive limited treatment here, emphasis being placed 

 on recent experimental work on active absorption and the forces opposing 

 absorption. 



It has long been recognized that water uptake by roots ma> be active 

 or passive (Renner, 1915; Kramer, 1945). Active absorption results 

 from a metabolic functioning of the roots and is evidenced by root pres- 

 sure, xylem exudation from excised roots and under some conditions, gut- 



