PHYSIOLOGICAL 313 



cited — each and every other theory must still be taken into account. 

 Thus Hales' initial and classic experiment, showing the rapid up- 

 pushing of water from a vine-stock cut so close as to have no leaves 

 at all, demonstrates the existence of his vis a tergo, especially, of 

 course, in the early growing season, when leaf-buds are thus being 

 helped (indeed forced) to open. Moreover, this root-push loses its 

 mystery, when we find in it the osmotic pressure produced by the 

 transformation of its insoluble reserves to plant-food in solution. 



In such ways, then, each of the many ascent-theories has to be 

 reckoned with, and for all that it may be worth; and, indeed, who 

 can say that no further factor of ascent may still remain to be 

 disclosed ? Even the descent of sugary sap, though small in quantity 

 and apparently slow also, as compared with the enormous and rapid 

 ascent of watery-sap, must also be recognised as a minor and in- 

 direct contribution towards necessitating the ascending current. 

 And thus so far it extenuates the failure of Hales' initial theory 

 (induced by mistaken analogy from Harvey's proof of the circula- 

 tion of the blood), and surviving to this day in popular language — 

 as out-of-date scientific theories so often do — as "the circulation 

 of the sap", though "the ascent of sap" has long been seen as the 

 essential process, and problem. 



In such ways, then, we see the necessity and the promise of 

 continued investigations, and these more and more comprehensive ; 

 since needing to be pursued into the analysis and comparison of 

 sap-movements in all kinds of vegetation, from simple to complex, 

 small to great, and in all their life-phases; and these again through- 

 out the changes of day and night, of temperature and pressure, of 

 season and climate, and of material environment — in all their 

 aspects, favourable or hostile. Much towards this has, indeed, 

 already been done, but necessarily as yet too sporadically. Revision 

 and co-ordination, as in so many other fields, are here needed 

 before we can understand sap-movements as comprehensively as 

 physiologists and physicians have been mastering their problem 

 of the circulation of the blood. In this progressing synthesis — 

 doubtless calling for the co-operation of many laboratories in many 

 lands — the various special viewpoints and theories will find due 

 place; and this doubtless in varying proportion, as with that of 

 Hales for the vine's growth-season. Our successors will thus at 

 length be able to admire this varied and varying play of Nature's 

 green fountains with clear intellectual conscience, and throughout 

 all her enchanted woods over the wide sunlit world. 



Transport of Foodstuffs. — In a typical plant the transpira- 

 tion current or "the ascent of sap" is obviously necessary to bring 

 the food-materials of water and salts, and dissolved carbon dioxide 

 as well, to the seat of photosynthesis, which in most cases is in 

 the green leaf. The question now arises: bow the CcU^bohydrates 



