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" The roots and the leaves are the chief, and, in many cases, the 

 only feeding organs of the plant. The roots imbibe water from the 

 soil by means of fine fibrils and root-hairs, the older, thicker portions 

 having no such faculty of absorption, but serving merely as conduits 

 and holdfasts. The water which exists in and amongst the particles 

 of the soil dissolves certain of its ingredients, so that when it enters 

 the roots it is not absolutely pure, but holds in solution a small 

 quantity of gaseous as well as of earthy or mineral substances. These 

 are required in the building up of the plant's substance, and in the 

 formation of its secretions. The way in which this solution or earthy 

 and gaseous matter is absorbed into the tissues of the roots has now 

 to be explained. It has been shown that, when a bladder containing 

 some thick liquid, such as syrup, is placed in a vessel of some thinner 

 fluid, such as ""water, there is a passage of the thinner liquid through 

 the membrane into the interior, so that the thick liquid becomes 

 diluted and the bladder stretched. This i.^ precisely what takes place 

 in the case of the roots. The thin solution of earthy matter passes 

 through the membranous walls of the root cells, there to mingle with 

 the thicker protoplasm which they contain. This process of absorption 

 is technically called osmosis, or endosmosis. 



" Eoot-absorption is probably always going on more or less, but 

 it is infinitely more rapid and abundant when a plant is in full growth. 

 The fluid when absorbed by the roots receives the name of ' sap.' We 

 know, by observation and experiment, that this sap rises from the 

 root, passes up the stems, through the branches, and enters the 

 leaves. The sap, then, flows upwards, and it is a matter of great 

 interest to ascertain how it is that such a fluid should ascend against 

 gravity." [No thoroughly satisfactory solution of the problem has 

 yet been arrived at.- — F.M.S.'] " The explanations are manifold — 

 several causes co-operate to bring about the result. In the first 

 place, the process of osmosis begun in the root-cells, is continued in 

 the young portions of the stem. Moreover, there now comes into 

 operation a process of diffusion, by virtue of which certain liquids pass 

 through others. Graham, an English chemist, called the thin, readily 

 diffusible liquids, ' crystalloids,' the thicker, less easily diffused fluid, 

 ' colloids,' from their gluey or gummy nature ; and he demonstrated 

 that the crystalloid fluids pass through and diffuse themselves amongst 

 the colloid ones. When the leaves are fully expanded another 

 circumstance helps powerfully to promote the rise of the sap, and this 

 is the profuse perspiration or evaporation of watery vapour and fluid 

 from their surface. Let a few leaves be gathered and placed under a 

 tumbler exposed to the sun, and shortly will be seen a quantity 

 of water condensed on the sides of the tumbler, which has been 

 evaporated from the leaves. This outflow takes place to an enormous 

 extent under favourable circumstances, varying in amount according 

 to the pressure, temperature, and moisture of the atmosphere, the 

 quantity absorbed by the roots, and the structure of the leaf itself. 

 There are thus an influx through the root, an upward current through 

 the stem, and an outflow from the leaves. All these act and re-act 

 one on the other ; the circumstances that favour the one for the most 

 part influence the others. If tlie one or the other be in excess, the 

 plant suffers. If the outflow from the leaves be greater than the 

 influx from the root, the plant withers, and unless the balance be 

 restored it will die. If the outflow 'be stopped while the influx 



