SCIENCE 



NEW YORK, MAY 5, 1893. 



THE SO-CALLED "SAP" OF TREES AND ITS MOVE- 

 MENTS." 



BY CHABLES R. BARNES, PROFESSOR OF BOTANY, UNIVERSITY OP 

 WISCONSIN, MADISON, WIS. 



The subject which I have chosen to present to you this evening 

 is not chosen so much on account of the information which I am 

 able to impart as for the purpose of correcting a great deal of mis- 

 information which is widely prevalent. Many false ideas as to 

 the nature and movements of what is popularly known as the 

 sap of trees are extant, and in a large number of cases these ideas 

 are founded upon mistaken notions of the physiology of plants. 

 Our own knowledge about many of these matters is yet exceed- 

 ingly imperfect, and it is for that reason that many of my 

 statements will of necessity be negative. The subject also is one 

 which must have considerable interest for those who are so inti- 

 mately engaged in cultivating fruit and shade trees as are the 

 members of this society, and I take it that no fact in regard to 

 the life and mode of working of the plants with which we are so 

 constantly dealing will be entirely without interest. 



What is Meant by "Sap?" 



It will be necessai-y for us at the outset to gain some accurate 

 idea, if possible, of what is meant by the word "sap." If we 

 think for a moment of its various uses, we shall see that it is a 

 word which designates not a fluid of definite composition, but 

 one under which is included a great variety of watery solutions. 

 The sugar-maker begins even before the snows have left the 

 ground to collect from wounds in the trunk of the maple-trees a 

 sweetish liquid which he calls "sap." After a considerable time 

 the proportion of sugar which this liquid contains diminishes 

 very greatly, and he then abandons his work because, as he says, 

 the "sap" has become too poor. The man who has postponed 

 pruning his grape-vines or trees to too late in the season finds that 

 from the cut surfaces a watery substance is trickling which he 

 calls "sap." But the sugar-maker will be unable to obtain either 

 sugar or syrup from this fluid, which is, however, called by the 

 same name as that from which he manufactures his sweets. 

 When a boy, who is making a whistle, hammers the bark of the 

 twig in the spring, he finds it easy to separate the bark, because, 

 as he says, the surface of the wood is then slippery with "sap." 

 The sap of the boy is widely different from the sap of the pruner 

 and the sap of the sugar-maker. 



Again, what we do not call sap may furnish us with some illus- 

 trations of the diversity of meanings of this term. We do not 

 ordinarily speak of the " sap " of the apple, or of the " sap " of 

 the grape, or of the " sap " of the orange, but call the fluids which 

 these fruits contain " juice." And yet they are not more different 

 in their composition from those fluids which we do call sap, than 

 the three examples already mentioned are different from each 

 other. We might therefore, in all reason, apply this word sap to 

 the juices of fruits. 



We popularly distinguish the older hard internal wood of the 

 tree under the name of " heart wood," from the younger, softer, 

 and lighter-colored external wood, which we call the "sap wood." 

 To the fluids which saturate the sap wood we are constantly in 

 the habit of applying the word "sap," but I have never heard it 

 applied to the exactly similar fluids which saturate the heart 

 wood. As far as the composition of these fluids is concerned, 

 there is no reason why that in the heart wood should not equally 

 well be designated as sap. 



' An address delivered before the State Horticultural Society of Wisconsin, 

 February, 1893 ; stenograpWcally reported, and revised by the author. 



What then are we to understand by the word "sap?" Evi- 

 dently not a substance of any definite composition ; but the word 

 signifies only in the most general way the various watery fluids 

 which are found in the plant. There is no reason indeed why 

 these solutions should not be called water, for in many cases they 

 are almost as pure as the water which we drink. In the chemist's- 

 sense, the water which we draw from our wells is a watery solu- 

 tion of various substances, and yet we do not designate it com- 

 monly by any other term than simply " water." In a similar 

 manner, it is quite proper for us, and perhajw it would conduce 

 to clearness of ideas, to designate the watery solutions in plants 

 simply by the term " water," understanding it in its popular and. 

 not in its strictly chemical sense. 



Movements of Water in Trees. 



Let us turn now to the consideration of the movements which- 

 the water in trees exhibits. I shall confine my remarks to trees 

 simply for the reason that they present the greatest variety of 

 water movement, and at the same time furnish the greatest difH- 

 culties in the explanation of these movements. If, therefore, we- 

 understand the movement of water in trees, we shall be able 

 readily to transfer these ideas to the movement of water in the 

 smaller plants, although the statements applicable to the trees are 

 not always applicable to the smaller plants, because of their 

 greater simplicity; however, the greater includes the less. 



The Evaporation Stream. 



In the first place, there is need of a very considerable amount 

 of water to supply the constant evaporation which is going on 

 from the leaves of trees. Immense areas of delicate tissue are ex- 

 posed to the dry air, and oftentimes to the hot sun, in the form of 

 foliage, and from this foliage there is going off at such times 

 large quantities of .water in the form of vapor. The water needed' 

 to supply this evaporation must come from the soil, because it is: 

 not possible for the leaves to take in any water, not even wheni 

 they are wet by the rains or by the dews. The water enters, not: 

 at the base of the trunk where the large roots are found, but only 

 at the extremities of the finest rootlets. At these points the root- 

 lets are clothed with a "nap" or "pile" of fine hairs. These 

 root-hairs must not be confounded with the fine branches of the 

 root, for it is only the finest branches which are covered with the . 

 close-set hairs. Consequently, it is only the youngest and most, 

 delicate parts of the root which allow the entrance of the water; 

 But the water escapes from the leaves, and from the point of en- 

 trance to the point of exit is a far cry for the coursing droplets. 

 How does it pass through this long space ? 



It is just here that our knowledge is most defective. We know 

 a number of things that are true in regard to it, and we know a 

 number of things which are not true in regard to it. 



We know that it moves in the sap wood of the tree, and 

 neither in the bark or in the heart wood. Many of you must 

 have made observations which are sufficient to establish this point. 

 You have, for instance, observed that the bark of trees might be 

 peeled off for a considerable distance, and that the leaves would! 

 still retain their green color and their freshness. In many cases^ 

 indeed, the mere removal of the bark from the tree is not suffi- 

 cient to bring about its death until several months, and in some- 

 trees not until several years, after the injury. Death, however,, 

 is inevitable sooner or later; but the fact that the leaves remaini 

 fresh for so long a time is evidence that the supply of water is not 

 interfered with. Death ensues from a totally different cause,, 

 namely, from the starvation of the roots in a way which will be- 

 explained later. 



Again, you must have observed that it is quite possible to have- 

 the entire heart wood of the tree removed, asisoftendoneby decayr 



