312 LIFE : OUTLINES OF GENERAL BIOLOGY 



the vessels with melted paraffin or gelatine, which solidifies when 

 it cools. In such cases the leaves of the shoot soon wilt. While one 

 must not conclude that no water can pass up in the walls of the 

 vessels, the main path is certainly in the cavities themselves. 



ASCENT OF SAP. — The essential question is — how does the water 

 rise, against graxity, and to the tree-top? — a height which even in 

 ordinary trees so much exceeds one or two full pump-lifts, and in 

 really lofty ones a good many more, even to the equivalent of ten 

 or even a dozen atmospheres? Many answers have been suggested, 

 but we cannot here go into all these. Indeed, it would need a volume 

 to do them full justice, since ranging through two centuries of 

 speculation and investigation, as from Hales to Bose, and even 

 onwards. Witness the former's root-pressure theory, the atmo- 

 spheric-pressure theory, the capillarity theory, the "relay-cells" 

 theory, and so on, to Bose's recent intra-cellular pulsation-lift 

 theory, as we may call it; each of which has been ably argued for, 

 and more or less experimentally supported as well. Return, then, 

 to look once more into the essential phenomenon, which each and 

 every theory has sought to explain: That — not simply by passive 

 evajxyration, but by active transpiration — much water passes of! 

 from the leaves as water- vapour ; so to keep up this vast expendi- 

 ture is the fundamental condition necessitating the corresponding 

 ascent of sap. And since the wood-fibres and wood-vessels form a 

 series continuous from root-points to leaf-tips, we have thus to 

 visualise what are practically long water-columns, albeit in more 

 or less transverse continuity' also. But for familiar physical reasons, 

 these columns are very hard to break, though easy to bend, so that 

 as Hcrlx?rt Sjx^ncer pointed out long ago (another theory !) the very 

 swaying of the wind, instead of hindering, must more or less help 

 them onwards. The leaf-cells, transpiring so much water, must needs 

 replace this from the water-column in the closely adjacent wood- 

 fibres and vessels of the leaf; and so they cannot but be exerting a 

 ]X)tont pull upon the whole water-column below. They thus contri- 

 bute to its rise; but in what measure? Were this pull of the leaf- 

 cells merely a question of ordinary hydrodynamics, it would not 

 suffice beyond a single atmosphere or pump-lift : but — here another 

 contribution — these li\nng cells of the leaf-parenchyma and epidermis 

 have their osmotic properties modified by their work of transpira- 

 tion, and thus with their pull potently increased upon the coherent 

 water-columns Ixlow. Here, then, is the old ms a fronte, now devel- 

 oped as "the cohesion-theory ", as its foremost exponent, Prof. 

 Dixon calls it. To our minds this seems at least the central and 

 elemental interpretation, since osmotic pressures may rise to not a 

 few atmospheres, as most physiologists seem coming to agree. Yet 

 none the less — even as for Spencer's minor contribution above 



