594 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



proved, that the red and violet rays always give maxima, the green 

 rays being the most inefficient. Hence all experiments devoted to 

 loss of water by heat will not apply to transpiration, but to evapora- 

 tion. They are clearly distinguishable on very hot days, when lettuces 

 wilt, though the light is the same as on a bright, cool day. Hence, 

 we do not quite see why transpiration is to be regarded as " a 

 physical phenomenon " (p. 5) ; for it ceases as soon as the protoplasm 

 is dead, and evaporation alone occurs. 



A considerable portion of the book is concerned with refuting other 

 theories ; while chapter 4 deals with the Cohesion Theory 0] the Ascent 

 oj Sap in Trees. As far as capillarity and cohesion water in tubes of 

 a very small bore are concerned, there is nothing new in this itself. 

 If a tube (J inch diam.) be placed in water, one end being stopped, 

 and it be drawn out vertically, the water will not flow out at the 

 base. Hence, when water has been drawn up by root-pressure into 

 the tubular vessels of the wood to 10-15 metres (according to Sachs, 

 " Phys.," p. 275), then, as long as transpiration goes on, the water will 

 be presumably drawn up ahove the hmiting height of root-pressure, 

 which keeps the stem supplied. But w^e fail to see how "cohesion" 

 can cause the water to rise. As there is no exposure to the air, as 

 in the barometer, atmospheric pressure is excluded. 



The author's own words are : — " Cohesion Theory. — ^This assumes 

 that the water in the conducting tracts of high trees hangs there by 

 virtue of its cohesion . . . The adhesion of water to the walls of the 

 tracheae we have shown to be very great," &c. (p. 87), but the actual 

 cause oj movement, other than transpiration withdrawing it from the 

 tracheae in the leaves, does not seem to be clearly expressed. He 

 adds : " Atmospheric pressure can supply the evaporating cell at 

 most only up to a level of about 10-3 metres." But, the tracheae 

 being closed at both ends, how can the atmosphere assist in the pro- 

 cess ? He proceeds : "It follows that the water in the tracheae above 

 this level is at all times in tension." But, still, what provides for the 

 continuous flow of water to supply the enormous loss of water of a 

 large leafy tree on a sunny summer day ? 



There is a feature in xerophytic plants which may throw light 

 upon the question. They are often characterized by possessing 

 water-storage tissues, such as the pseudo-bulbs (swollen internodes) 

 of epiphytic orchids. In some desert plants, certain groups of large 

 cells round the vascular bundles are filled with water ; or it may be 

 the cortex, or roots, &c. Now all this water can be utilized in prolonged 

 seasons of drought. How is it passed into the tracheae ? Presumably 

 it follows exhaustion by contraction of the cells by their elasticity. 

 Now, what is excessive in cases Hke these may act normally in ordinary 

 trees and herbs. The cellular tissues, i.e. pith, cortex, and medullary 

 rays, are all filled with water, derived from the excess over transpira- 

 tion, supphed by root-pressure, capillarity, and tension. Transpira- 

 tion ceases at night, but the supply never stops. Will not any ex- 

 haustion in the tracheae be at once met by the elasticity of the turgid 



