24 BOTANICAL THEORY OF THE FLOW OF SAP. 



65. In those hot climates, the quantity of sap which flows in a short space of time 

 through the vessels of such a plant is incredibly great. In the month of April, 1834, 1 cut 

 a vine, which was growing wild on the edge of a forest in Virginia, asunder with one blow 

 of an axe ; the cut surface, which was about 1J inch in diameter, exhibited its open ves- 

 sels, from which there poured out an uninterrupted stream of ascending sap. In the 

 course of eight hours there was collected of this fluid seventy ounces, and this was 

 probably a far less quantity than would have been raised under ordinary circumstances, 

 where the leaves aided the spongioles by their exhausting and pushing action, in the 

 way which will be presently explained. As all persons know, in these warm climates 

 processes of vegetation go on with extraordinary rapidity, and in summer, from the in- 

 tense brilliancy of the sun and the high temperature, the midday assumes peculiarities 

 which in colder climates are never witnessed. In the forests of those countries, at that 

 hour, there reigns an unbroken silence, a period of complete repose. The wild dove, 

 which all morning long has poured forth her plaintive note from the top of some with- 

 ered pine, seeks for a shady covert, and plumes her feathers in the heat of the day ; 

 the wanton squirrel forsakes his gambols, and retires to his nest ; the turtle dozes on 

 the surface of the stagnant pools. There is not a cloud upon the sky, there is not a 

 breath in the air. The sunbeams tremble upon the leaves, or sparkle upon the sand, 

 or steal in long gleams of light across the water. A profound silence reigns among 

 mvriads of living things, inhabitants of those solitudes ; a silence only broken at inter- 

 vals by the rustling of the brown lizard in the leaves, or the distant tap of the lazy 

 red-headed woodpecker upon some hollow trunk. 



66. As we have said, if at the proper season of the year the stem of a plant be divi- 

 ded, ascending ^ap will copiously flow from the extremity of the stump. If, moreover, 

 the part which has been cut off, and which bears the leaves, has its cut extremity im- 

 mersed in a vessel of water, imbibition of that water will rapidly take place, and life 

 Be maintained for a time. From these facts, and others of a similar kind, botanists 

 have had no difficulty in recognising two distinct sources of action concerned in the 

 flow of the sap. They have shown that the spongioles or extremity of the roots, which 

 apparently consist of a lax cellular or spongy tissue, have the quality of impelling the 

 sap upward, and hence it flows from the extremity of a stem from which the upper part 

 has been cut off. From the fact that a branch on which the leaves still remain pos- 

 sesses the quality of removing water from a vessel in which its extremity is immersed, 

 they have regarded those organs as possessing a kind of suction power, due to the evap- 

 oration which takes place under the influence of heat from their superficies. The spon- 

 gioles, therefore, drive the sap upward, and the leaves, by their exhaustive effort, draw it. 



67. But this doctrine is essentially defective. It furnishes no reason for the down- 

 ward flow of the sap ; none for the well-known fact that the exhausting action of the 

 leaves is under the control of light. To a certain extent, it is true, an evaporation ta- 

 king place from the leaves will conspire in its result with the movement of the ascend- 

 ing sap, but it must not be forgotten that it will also antagonize with the movement of 

 that which is descending. That there are two seats of action in the phenomenon, the 

 spongiole and the leaf, is quite true ; but its causes a o very different from those cer.er- 



