544 



THE SOUTHERN PLANTER. 



to grow properly it must have food which 

 is favourable ; so that we may say there 

 is a kind of choice, which, as already re- 

 marked, is as it w^ere negative. It has the 

 power of refusing to grow unless proper 

 food is supplied. That is the only way in 

 which we can suppose that plants have 

 really a choice. I think this statement is 

 sufficient to ex})lain many of the phenom- 

 ena- which have been brought forward, 

 as tending to prove the existence of 

 choice food in plants. The circumstance 

 that a fair supply of food favourable to 

 the plant exists in the soil is sufficient to 

 account for the plant possessing abundance 

 of that particular substance in its ash, and 

 a greater abundance of that substance in 

 its ash than another plunt whose constitu- 

 tion does not require that particular kind 

 of food, and which has been well devel- 

 oped in a different soil In addition to 

 this refusal to produce organization out of 

 unfit food, we have certain phenomena 

 which are partly chemical and partly phy- 

 sical. The absorption of the root depends 

 to a great extent upon what is called en- 

 dosmosis — the power of the membrane of 

 the rootlets to draw in fluids and solutions 

 by which the root is surrounded with a 

 certain amount of force, arising in many 

 cases simply from the fact that the fluids 

 within are denser than those without. 

 But decompositions probably take place 

 immediately inside the membrane of the 

 rootlets, and the decompositions may 

 cause difl'erences in the proportional ab- 

 sorption of different constitutions of the 

 soil. That, however, is rather a specula- 

 tive than an assured point. The greater 

 part of the absorption of liquid food is de- 

 cidedly a mere physical process. Suppo- 

 sing the food to be favourable and acces- 

 sible, and supposing the proper conditions 

 to be fulfilled in the different species, the 

 absorption of food is, to a great extent, a 

 mere physical process. It is the result of 

 the action of endosmosis. If you put a 

 solution of gum into a bladder, and place 

 that in water, the gum will attract the wa- 

 ter with great energy, so that it will swell 

 out, and if the proportions are suitable, 

 may even burst the bladder. If the pres- 

 sure is withstood there ma}^ be a filtration 

 through the bladder from the tension pro- 

 duced by the excess of absorption. Some 

 experiments have been recently made by 

 the German physiologist, Hofmeister, 



showing that the endosmosis is the princi- 

 pal cause of the flow of saj> upwards into 

 plants. More than 150 years ago Hales 

 showed that such sap flowed out from 

 plants, especially from the vine in spring, 

 with considerable force. His experiments 

 with glass tubes containing mercury 

 showed that cut branches of the vine 

 emitted the sap in spring, at the time of 

 what is called the bleeding of the vines, 

 with such force as to raise a column of 

 mercury equal, in some instances, to an 

 atmosphere. The same observation has 

 been confirmed by other observers ; and 

 Briicke has observed that the force de- 

 pends upon the distance of the branch 

 from the root — for instance, that a branch 

 close to the root would lift 30 inches of 

 mercury, while a branch 15 inches above 

 the root would only lift half that quantity ; 

 so that the branches and the stem acted, 

 as it were, like intercommunicating tubes, 

 and the pressure was diminished in pro- 

 portion to the distance from the roots. 

 Hofmeister has gone further than this, and 

 has shown that the force lies in the roots. 

 By fixing the tubes upon the roots them- 

 selves, and in making some experiments 

 on the common herbaceous garden plants, 

 he has found that the same force exists 

 throughout all of them, and throughout all 

 seasons, modified by conditions of humid- 

 ity of the atmosphere and soil. In one 

 experiment on the common foxglove, a 

 plant a yard high was cut off near the root, 

 and a tube containing mercury, similar to 

 a barotneter tube, was fixed upon this ; it 

 was found that the force of the sap driven 

 out from the crown of the root by the ab- 

 sorption of water from the surrounding 

 soil would raise a column equal to 20 

 inches of meicury. Even little seedling 

 peas were found to be capable of forcing 

 up a column of 1 inch of mercury. The 

 structural conditions of the tissue of the 

 root all tend to show that these experi- 

 ments are worthy of credit; the condi- 

 tions of the root are exactly those which 

 would favour this endosmosis, and also 

 this driving of the fluids upwards in the 

 long tubes and canals of the woody tissue, 

 w^hen it was filtered out from the absorb- 

 ing cells by the tension produced by ex- 

 cessive absorption. In these experiments 

 a most important difference in the pres- 

 sure was found to result from the amount 

 of evaporation going on in the leaves 



