240 



SCIENCE. 



[Vol. XXI. No. 535 



and yet to have the leaves remain fresh and green for an indefinite 

 time. In fact, the rotting-out of the heart wood scarcely inter- 

 feres with the vitality of the tree, except as it renders it mechani- 

 cally weaker, and consequently more liable to be overthrown by 

 storms. If any further proof were needed, it is perfectly possible 

 to show experimentally that the sap wood alone is engaged in the 

 transfer of the water required for evaporation by cutting into it. 

 A saw-cut which passes through the sap wood, but leaves the 

 heart wood intact, brings about within a very short time the with- 

 ering of the leaves. In some trees, indeed, a cut which severs 

 •only the outer youngest layers of the sap wood will produce the 

 same effect, since in such trees only the youngest layers of the 

 -wood carry the water. By experiments on twigs it can be de- 

 monstrated that withering will occur even if the bark is almost 

 completely uninjured. 



We know the water to supply evaporation moves chiefly in 

 the cavities of the elements of the wood. The wood of the tree 

 is composed of a large number of fibres, that is, elongated cells 

 pointed at both ends, and of duds, that is, tubes of great length 

 formed by the breaking together of rows of cells placed end to 

 end. You can get an idea of the manner in which these ducts 

 are formed by imagining a series of round pasteboard boxes piled 

 one on top of another, after which the top and bottom of each is 

 removed, so that, instead of a series of separate chambers, we 

 have now a long tube. The fibres may be likened to a series of 

 lead pencils, sharpened at each end, and placed in contact with 

 each other, the points of the lower ones overlapping the next ones 

 above and fitting in between them. In my illustration the cavity 

 of the fibre would be represented by the lead, and it would be 

 more accurate if we could conceive of the cavity as not extending 

 entirely through the pencil, but stopping short of the point. 

 Minute pits extend from the cavity of one of these fibres to the 

 other, and the walls also of the long ducts are also marked by 

 larger thin spots. It is in the cavities of these ducts and fibres 

 that the water chiefly travels. 



We do not know what part is taken in this ascent of the water 

 by those peculiar elements of the wood which you know by the 

 name of silver grain or the pith rays. You will remember these 

 as the shining plates of tissue which extend from the centre of 

 the wood toward the circumference. They are particularly 

 prominent in the oak and show most when it is split " with the 

 .grain." It is probable that these cells have a great deal to do 

 ■with the movement of water, but their exact role is not fully 

 agreed upon. 



We are in almost total ignorance at the present time as to 

 the force by which the water is elevated through so many feet. 

 There are trees in the gullies of Victoria, Australia, whose height 

 exceeds 470 feet, and we must Invoke some force which is able 

 to raise water from the level of the soil to the level of the highest 

 leaf. A year ago we thought we had a hypothesis which would 

 account for this movement, but later researches have brought to 

 light some facts which are at present totally irreconcilable with 

 what was a most charming, and, at that time, a most satisfactory 

 ■explanation, and we shall be obliged to abandon it unless the wine 

 •of the new knowledge can be held by the old bottles of theory. 



At the time when our knowledge of capillarity was greatly ex- 

 tended by the celebrated researches of Jamin, it was thought that 

 we had knowledge of a force adequate to account for the raising 

 of water to these great heights. The fibres and ducts which I 

 have described to you seemed to answer very perfectly the re- 

 quirements of capillary attraction, and it was thought that this 

 force, bv reason of which water rises through narrow spaces, was 

 the one sought. But the rise of water in capillary spaces is pro- 

 portioned to the size of the opening; the smaller the opening, the 

 higher will it rise. With the decrease of the calibre of the tubes, 

 however, the friction increases enormously, and only small quan- 

 tities will be able to be moved on account of the diminished size 

 of the tubes. It was quickly seen that, in order to account for a 

 rise of even a hundred feet, the tubes of the wood must be vastly 

 smaller than they really are. 



When it was found that the air in a plant is under a less press- 

 ure than that outside the plant, it was thought that the force had 

 been discovered, and that atmospheric pressure furnished the ex- 



planation. Negative pressure, however, on the interior never 

 reaches zero, and consequently cannot account for a rise of more 

 than 33 feet. 



Again, what was called root-pressure was invoked to explain 

 the phenomena. It is found that water is absorbed at certain 

 times so rapidly by the roots that it exists in the plant under con- 

 siderable pressure, and it has been claimed that root-pressure, com- 

 bined with the other forces already known, was adequate to ac- 

 count for the rise of water. But this, too, has failed us. 



It is perhaps the greatest weakness of the last theory (that of 

 Godlewski), which we have just had to abandon temporarily at 

 least, that it depended for its explanation upon the indefinite and 

 iUusive " vitality "' of certain portions of the plant. Godlewski's 

 brilliant hypothesis, which ascribes to the activity of the living 

 cells of the medullary rays the function of receiving from lower 

 levels the water and passing it on to higher tissues through 

 rhythmic variations in their osmotic power, due possibly to respira- 

 tory changes, may yet hold the clue which we are seeking. But 

 when Strasburger jacketed a young tree for a distance of S.'i feet, 

 and kept it surrounded by hot water until all of the living cells in 

 the tree trunk were unquestionably killed, and when under these 

 circumstances the water-supply to the leaves was not interfered 

 with, so that they remained green and fresh, we were obliged to 

 conclude that the lifting of the water is not dependent upon the , 

 life of the tissues directly, but that it is evidently carried on by a 

 physical process yet to be explained. 



Before passing from this topic of the movement of water which 

 supplies evaporation, I must allude to a very common and wide- 

 spread idea, — at least I judge it to be widespread, because it is so 

 frequently propounded by my students, — that "the sap goes down 

 in winter and up in the spring." Just where the sap is supposed 

 to go in winter is not exactly clear; since, if the roots are absorb- 

 ing water in the fall when the evaporation is diminished, they 

 are likely to have quite as much water as they can hold already. 

 The conception, apparently, is that all of the water lodged in the 

 trunk and spreading branches goes downward into these roots. 

 It needs, however, only the most casual examination of trees in 

 winter to discover that at this time they are almost saturated with 

 water. The twigs of the hickory tree, for example, will be frozen 

 on a cold day in winter, so that they are as brittle almost as 

 glass, and one can snap off a twig half an inch in diameter as 

 though it were an icicle. The same twig, when not frozen, on a 

 mild day will be so tough that there will be no possibility of 

 breaking it. 



Again, if one cuts off a branch from a tree in winter and brings 

 it into a warm room, he will quickly discover that water is oozing 

 from the cut end, showing that the twigs are almost saturated 

 with it. As a matter of fact, the water in trees increases from 

 mid-summer or early fall to the beginning of growth in early 

 spring. There is thus no necessity for any " going up " of the sap 

 in spring until the leaves are expanded and the water with which 

 the tree is already saturated begins to be evaporated from the 

 foliage. 



Bleeding. 



A second movement of water in trees is that which occurs in 

 the so-called "bleeding." The bleeding of trees occurs at different 

 times of the year, either before growth has begun at all, or just 

 as it is beginning. In the two cases the cause is quite different. 

 We find a good e.xample of both sorts of bleeding in the gathering 

 of the sap by the sugar-maker. This gathering begins at the 

 time when the ground is still frozen and the roots are almost or 

 quite unable to absorb any water, but at a time when the air is 

 warmed through the middle of the day by the increased heat of 

 the sun. At first the expulsion of water from wounds made in 

 the trunk is due to the expansion by heat of the air inside the 

 smaller branches and twigs of the tree. This sets up at once a 

 pressure upon the water, and this pressure is transmitted to all 

 parts of the tree. The water with which the tree is filled is 

 thereby forced out as soon as an opening is made for its escape. 

 Later in the season, however, the roots begin their work of ab- 

 sorption, and there is then set up the so-called root-pressure, by 

 reason of which the water is forced out at the same openings. 



