22 FLOW OF THE SAP AND CIRCULATION OF BLOOD DUE TO THE SAME CAUSE. 



CHAPTER II. 



ON THE MECHANICAL CAUSE OF THE FLOW OF THE SAP IN PLANTS. IT IS DUE TO THE 

 CARBONIZATION OF WATER IN THE LEAVES BY THE LIGHT OF THE SUN. 



CONTENTS : The Flow of Sap and Circulation of Blood are probably due to the same 

 Physical Cause. Amount of Water circulating in Plants. Botanical Theory of the 

 Flow of Sap fails for the descending Sap. 



Capillary Attraction described. Elevation or Depression of Liquids depends on their 

 wetting or not wetting the Tube. No Flow in an ordinary Capillary Tube. Con- 

 ditions for producing a Flow such as Evaporation, Decomposition, and Solution. 

 Endosmosis produced on these Principles by Solution. Dutrochet's Experiments. 

 Explanation of them. General Law of these Movements. Force with which they 

 take place. Capillary Attraction due to Electricity. 



Application of these Principles to the Ascent of the Sap. Exhausting Action of the 

 Leaves. Cause of the Descent of the Sap. 



The Light of the Sun is the Cause of the Flow of the Sap both in its Ascent and 

 Descent. 



58. IN the lower classes of plants, such as those of which we have been speaking, 

 which carry forward absorbent processes on every part of their surface, the mechanism 

 for nutrition and respiration is of the simplest character, and, as we shall hereafter see, 

 is nothing more than a surface action ; imbibition, nutrition, and aeration all taking 

 place upon the same point. But in any organized structure, as soon as types of cen- 

 tralization are adopted, and specific processes carried on in distant parts, the nutritious 

 juice must necessarily pass from place to place, and undergo in its route proper chemi- 

 cal changes. In its movements it must flow along predetermined channels, and take in 

 succession given directions. To accomplish this a circulatory apparatus is required, 

 adapted in each instance, as to form and character, to each peculiar organism. In the 

 higher classes of animal life, this process of movement passes under the designation of 

 the circulation of the blood ; in plants, under the designation of the flow of the sap. 



59. There is that unity of plan in all the works of Nature which causes us at once 

 to understand that in these various mechanisms the same physical principles are resorted 

 to ; that the flow of the sap and the circulation of the blood are due to the same 

 powers. A theory of such movements, therefore, can only be true when the principles 

 which it involves give at once a clear account of every case, of the flow in plants and 

 in animals, and even in every individual instance in each of these great classes of 

 organized beings. Such a theory should be applicable to the movements in flowering 

 plants, and to all its various modifications in the less complicated orders of vegetable 

 life ; for each particular instance it should show why specific apparatus is required, and 



