THE MATERIAL OUTGO OF PLANTS 349 



Ascent of water. — As to the forces concerned in the ascent of water, 

 little that is definite can be said, for the problem is one of extraordinary 

 complexity, and knowledge of the exact physical conditions is very 

 difficult to attain. Nor is it likely that the problem could be solved were 

 all the factors in the plant body known, simply for lack of knowledge 

 of the physical principles involved. 



Capillarity. — Some " causes " frequently assigned and popularly 

 current may be definitely discarded. The first of these is capillarity, 

 as commonly understood. The xylem ducts are narrow tubes. Water 

 rises in capillary glass tubes above the level outside, and the smaller the 

 bore the higher it rises. Oil rises in a twisted lamp wick by capillarity. 

 What more simple than to " explain " the rise of water in the ducts of 

 the xylem strands by ascribing it to capillarity, since here are " strands " 

 and " tubes "? But surface tension (which is a better name for capil- 

 larity) implies a free surface, and within the duct there can be no free 

 surface which is lifting, as in an open glass tube. If one appeals to the 

 surfaces bounding the bubbles of gas so common in tracheae (see p. 350), 

 it must be remembered that for every meniscus concave upwards there is 

 one concave downwards to balance it. Nor can one neglect the numer- 

 ous transverse walls in the xylem of angiosperms, 1 and the fact that all 

 the effective xylem of gymnosperms is composed of tracheids. How 

 surface tension forces may operate at the evaporating surfaces in the 

 leaves is not known; but these are not the ones referred to when capil- 

 larity is invoked as the cause of the ascent of water, or at least an aid to 

 it. 



Root pressure. — Root pressure (see p. 336) is frequently alleged to be 

 active in forcing water up; and it is even held to be adequate in the case 

 of the herbaceous plants and low shrubs, though confessed to be insuf- 

 ficient in the taller trees. The radical difficulty with turgor in the root 

 cortex as a cause of the ascent of water, or at least an aid to it, is that it 

 does not exist when it is most needed. In the very nature of the case the 

 root cortex can be fully turgid only when it has an abundance of water; 

 and it is not likely to have that when evaporation is active. To develop 

 root pressure it is necessary to check evaporation, as by decapitation, 

 and only after a time does water begin to ooze from the xylem in conse- 

 quence of turgor. Often water at first enters the stump of a decapitated 

 plant, showing clearly that there was no surplus of water under previous 



1 The longest continuous ducts found exceed 5 m., but those 1 m. long are rare, and 



the average is probably less than 10 cm. 



