PHYSIOLOGY 187 



duction of the water, and, also, of determining the maximum velocity of the 

 transpiration current, from observations based on the path and rate of movement 

 of-a coloured solution taken up by » plant, is not free from objection ; for the 

 colouring matter would not pass through the stem at the same rate as the water 

 in which it is dissolved, but would be drawn out and held back by the cells. The 

 employment of coloured solutions will, however, be found instructive for merely 

 showing the course of the transpiration current. The transparent stems of the 

 Balsam, Impatiens parviflora, and the white floral leaves of Lilies, Camellias, 

 Mock Orange, etc., in which the coloured vascular system will stand out as a fine 

 network, are especially adapted for such an experiment. 



In water-plants and succulents, in which little or no transpiration 

 takes place, the xylem is correspondingly feebly developed. In land 

 plants, on the other hand, and especially in trees with abundant 

 foliage, the wood attains a much greater development. All the wood, 

 however, of a larger stem does not take part in the task of water- 

 conduction, but only the younger, outer rings. Where there is a 

 distinction between heart- and sap-wood, under no conditions does the 

 heart - wood take part in the conduction of the water, which is 

 transferred exclusively by the younger rings of the sap-wood. 



The character of the forces which cause the ascent of the trans- 

 piration currents is still unexplained. Transpiration itself only makes 

 a place for the inflowing water ; it does not furnish the force which is 

 necessary to rapidly convey a large volume of water for a considerable 

 distance through the wood. Every operation by which work is 

 accomplished implies a corresponding expenditure of force ; and the 

 force which is capable of raising great masses of water to the tops 

 of a tall Poplar or of a Eucalyptus 150 m. high, must be consider- 

 able. But, as yet, all efforts to determine the nature of this force have 

 been fruitless, and all previous suppositions have been shown to be 

 untenable.- ; 



It has been already explained that the" root-pressure cannot exert such a 

 force during transpiration (p. 184). 



Osmotic forces act too slowly to be of any value, and, moreover, there is no 

 fixed distribution of osmotic substances that would account for such a current. 



The transpiration current cannot be due to capillarity. In the first place, con- 

 tinuous capillaries are entirely wanting in some plants (the Conifers, for example), 

 and in the stems of others they are only present for comparatively short distances. 

 Secondly, the concave menisci in the elements of the wood are not in relation with 

 any level or convex surface of water, in which case alone they could have 

 effect. Thirdly, the height to which liquids can rise by capillary attraction, and 

 it would be less in the vessels and tracheids than in a glass tube, does not approach 

 the height of an ordinary tree ; and, finally, the rate of ascent induced by 

 capillarity decreases so greatly with the increasing height of the fluid, that so 

 copious a flow of water as occurs in plants would be impossible. 



Atmospheric pressure has, also, been shown not to be the cause of the trans- 

 piration current. It is true that the vessels and tracheids of vigorously transpiring 

 plants contain rarefied air between the short columns of water. This is evident 

 from the way in which stems cut under mercury become penetrated by it. But 



