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EVOLUTION 



May, 1932 



The Amateur Scientist 



A Monthly Feature by Ai.lan Broms 



THE SPRING SAP 



IN REFERRING to the Spring Sap, 1 do not 

 mean either the spring poet nor the 

 young man whose fancies fondly turn at this 

 time of year. I refer to something far more 

 prosaic, the sap which the warmth of spring 

 turns loose within the plants. Once it begins 

 to flow, the leaves bud and the flowers 

 bloom, — but I must be careful lest I wax 

 poetic myself. Like the blood within the 

 animal, the plant sap is the bearer of the 

 stuffs of life. I put "stuffs" in the plural 

 because there really are several, mineral salts, 

 nitrates and so on, soaked up with water 

 from the earth through the roots and borne 

 aloft into the green leaf tissues where the 

 real food factory is. There the green stuff, 

 the chlorophyll, absorbs the sunlight and 

 employs its energy to tear apart the carbon- 

 dioxide taken from the air, releasing the 

 oxygen and then combining the carbon with 

 water to form sugars and starches which 

 store the sunlight energy whose calories we 

 may later consume by eating these carbo- 

 hydrates. Once manufactured, these food- 



stulis are transported down into the plant 

 stem by means of the circulating sap, as you 

 can find out for yourself by tapping a sugar 

 maple tree or sucking a stalk of sugar cane. 

 Finally those sugars and starches may go 

 into storage, as in any starchy potato or 

 plump sugar beet. 



Why the sap circulates is a bit of a mys- 

 tery. To call the force, which lifts it through 

 the tubes of the plant stem, osmosis or 

 capillarity just adds big words to the con- 

 fusion. But we have an idea how the trick 

 is done. Those tubes have fluted walls, 

 little ridges inside running vertically along 

 their length, with little troughs between. 

 The sap does not fill the center of the tube 

 at all, but flows up those very narrow 

 troughs, each bit of the watery sap adher- 

 ing to the ridges alongside, thiu supporting 

 itself just as the water wetting the side of a 

 glass tumbler supports itself by sticking to 

 the glass surface. In other words, the sap 

 lifts itself by wetting the sides of the nar- 

 row troughs of the plant tubes. Up in the 



leaves, as we have just discovered, the water 

 of the sap is used in making sugars and 

 starches or is wasted by evaporation from 

 the leaf surface. That would dry out the 

 upper plant tubes except that the nearby sap, 

 by flowing in to wet the tube surfaces, re- 

 places the lost water. 



But how does that get the sugar and 

 starch products of the leaf factories to move 

 downwards against the upward current of 

 the sap? It does seem impossible, but you 

 can work that out too by means of an ex- 

 periment. Put some water into a long pan 

 and let it quiet down so it is perfectly still. 

 Then carefully put in some sugar or salt at 

 one end, disturbing the water as Uttle as 

 possible. Of course the sugar or salt dis- 

 solves, but does it also move through the 

 water to the other end? Touch your finger 

 gently to the water at the other end and 

 take a taste, at first it has no sweet or salty 

 taste. But wait a while, then try again, 

 you will find both ends of the pan of water 

 equally sweet or salty. The sugar or salt 

 has not only dissolved, but also diffused 

 itself throughout the liquid. The same 

 thing happens to the sugars and starches in 

 the leaf sap, they diffuse throughout all the 

 sap in the plant, doing so faster than the 

 flow of the sap the other way. At least that 

 is one guess as to what happens, though we 

 are not too sure. 



actual creation of life as one now knows it, for only with the 

 coming of chlorophyll to utilize the sun's energy did life on a 

 large scale become possible. 



Evolution of Proteins 



Since the colloids characteristic of life have as their basic 

 element the proteins, one of the most important of the first 

 steps in evolution was the evolution of the amino acids that 

 make ou the proteins. Only about a score of the great num- 

 ber of theoretically possible amino acids have been utilized to 

 form all the proteins that make up the living world. 



Presumably, vastly more than these few sorts of amino 

 acids have occurred in nature, yet for some reason they have 

 been discarded as unsuitable or urmecessary for the sorts of 

 proteins that are required to make a living organism. Nor are 

 all the few known amino acids of proteins necessary for each 

 and every protein, for no protein yet analyzed has been found 

 to contain all of them, and many proteins seem to contain 

 relatively few. It is perhaps significant that the proteins 

 most concerned with cell multiplication and heredity, the 

 nucleoproteins of the germcells, seem to have the smallest 

 number of amino acids, as if these amino acids were the essen- 

 tial ones to keep life going and reproducing, while the rest 

 represent merely those responsible for the differentiation and 

 the functions less essential than reproduction. 



It is most significant, furthermore, that much the same 

 amino acids are found in all living cells, whether simple bac- 

 teria and yeasts, or the more complex plants and animals. 



In view of the general principle that the individual in its 

 development tends to recapitulate the development of the 

 species, one might hope to find the various steps of the evolu- 

 tion of the protein in the substances produced when bacteria 

 or yeasts synthesize their own proteins from the simple mix- 



tures of salts that they are able to utilize in reproducing 

 themselves in vast numbers. But unfortunately this is not the 

 case. The reproduction is completed so rapidly that one can 

 not catch the different stages. 



Chemical Steps in Evolution 



One can not hope, therefore, to learn through chemical an- 

 alysis of growing cells the steps by which the proteins as they 

 exist now were developed. Even chemical study of that beauti- 

 ful material for the investigation of the evolution of the in- 

 dividual, the developing eggs of hens, has not yet thrown 

 any light on the problems of evolution, although there is room 

 for much more profitable study in this field. Undoubtedly, 

 closer biochemic study of developing eggs would give evidence 

 as to the evolution of chemical structures. The vitally im- 

 portant nucleic acids, which form so large a part of the hatch- 

 ing chick, are almost entirely new-formed from other com- 

 ponents during incubation. Can one not learn the chemical 

 evolution of keratin, as well as one knows the morphologic 

 evolution of bird's feathers from fish scales? If one could 

 follow the transformations that produce hemoglobin in the 

 egg, one would probably learn how it came into existence in 

 the Cambrian. Needham, who is one of the few biochemists 

 attacking the problem of the evolutionary significance of the 

 changes in the developing egg, brought out an interesting 

 bit of evidence of the well known relationship of birds and 

 coldblooded forms by showing that the developing chick and 

 dog-fish embryos alike have to synthesize 90 percent of the 

 scyllitol that they produce. There is, furthermore, the suggest- 

 ive discussion of the "Paleochemistry of Body Fluids and 

 Tissues" by Macallum, suggesting that the inorganic con- 

 stituents of one's body portray the composition of the pre- 

 Cambrian seas from which one's ancestors arose. 



