208 THE MOVEMENTS OF WATER 



climate may alter the shape and structure of individual plants, and even the 

 general habit of the whole vegetation. 



There is a certain optimal supply of water for terrestrial plants at which 

 growth and development are most active. When submerged, many plants 

 are incapable of any development at all, and very frequently the complete 

 saturation of the soil with water exercises a retarding influence upon 

 vegetative activity. Moreover, the infiltration of the intercellular spaces 

 with water is disadvantageous to most terrestrial as well as to most aquatic 

 plants, so that it is only in those of the latter which have no air-spaces that 

 the vegetative conditions are most favourable when they contain the highest 

 possible amount of water. Transpiration and the water-currents which it 

 induces are of importance in various ways, particularly in accelerating the 

 passage of the salts absorbed from the soil up to the summit of a tall tree. 



Each cell strives to absorb water until it is fully turgid, and hence 

 a stream will be originated in this manner which will extend from the 

 absorbing to the transpiring organs. In a multicellular fungal hypha this 

 water passes from cell to cell, but special channels become necessary when 

 water must be carried to distant organs to supply that which is lost by active 

 transpiration. Numerous researches have shown us clearly that where 

 differentiation of tissue and division of labour exist, the xylem portions of 

 the fibre-vascular bundles are entrusted with this function. How and by 

 what means the water is so easily raised and so rapidly transferred to the 

 summits of the tallest trees has not as yet been satisfactorily explained. It 

 is certain, however, that the water is not merely driven upwards from the 

 roots or base of the stem by the root-pressure acting like a force pump, 

 but that the removal of water from the conducting channels exerts a force 

 transmitted backwards as far as the absorbing organs, causing in these 

 a corresponding entry of water (Sects. 34, 35). 



It is only when transpiration is feeble and a large amount of water 

 has accumulated that fluid exudes from a decapitated root-stock or from the 

 end of a cut branch. Such ' bleeding' is shown by cut stems of vines, beeches, 

 &c., in spring before the leaves unfold, and in summer also if transpiration 

 is prevented. The bleeding of a decapitated stump ceases after a time, 

 while if a plant is cut while actively transpiring, the surface of its attached 

 stump may at first absorb water. 



In the process of bleeding, water or a watery solution exudes from the 

 vessels and tracheides, and the amount lost in this manner may finally far 

 exceed the joint volume of the root-system and stump of the stem. The 

 water escapes from the cut surface because here the least resistance to its 

 exit is interposed. The continuity of the stream, as well as the pressure which 

 is indicated if a manometer is attached to the stump of the stem, shows that 

 forces are active in the interior of the stem, which drive the water into the 

 cavities of the vessels or other spaces, and so create internal positive pressure. 



