Since the law of gravity decreed that water should not run uphill, trees have played the role of recreating. In the trunks of the trees, the sap rises apparently in direct defiance of this edict. And since science insists on finding some rational explanation when well-defined laws of Nature seem to be violated, it has persistently tried to explain the hidden forces that cause the upward flow of this vital fluid in living plants. But despite patient search and investigation, there still exist in Nature many gloomy frontier countries, barely illuminated by our real knowledge of material things and barely defined by our most intelligent theories. The forces that lie imprisoned within the atom; the problem of the ultimate nature of matter, the ancient question about the origin of life itself – all this belongs somewhere in that unexplored and unexplored country for which science has found only indifferent guides.
It is well to admit that such enigmas still defy our most careful investigations, especially when they attempt to discuss what the great Darwin himself called “the most nebulous of subjects.” So familiar has this fact of sap flow rise become in our daily lives that only those who have attempted to explain its processes scientifically have realized its bewildering complexities. Popular misconceptions on the subject are numerous. Today, no less than twenty years ago, your practical woodsman talks about the sap being “up” in summer and “down” in winter, although in truth there is less sap in the trees during the warm months than at any other time.
In the case of “up” you have a rebuttal to the old saying that what goes up will surely come down; because much sap that rises never returns to the earth. Here, of course, I am using the word “sap” rather loosely, as ‘(usually done, to refer to whatever moisture a tree can get. In fact, the fluid that rises is pretty much the same as the water found in the soil felts and flows in streams, and only when it has been subjected to the mysterious chemical changes that take place in those green laboratories, can the leaves be strictly known as sap.
After these changes, it begins its downward course loaded with food matter: starches and sugars, cell builders and wood producers. And long before it reaches the base of the tree, where it started from, most of this material has been deposited and most of the water has evaporated. So the original fluid that goes up the tree never comes back down. And when in autumn the leaves fall and the sap ceases to circulate, it remains immobile in the trunk of the tree and in the branches, to remain there throughout the winter, waiting for the arrival of the leaves to put it back into action.
