10 



THE l-ARMER'S MAGAZINE. 



place immediately inside the memliraiie of the rootlets, and 

 the decorapoaitions may cause differences in the proportional 

 absorption of different constituents of the soil. That, how- 

 ever, is rather a speculative than an assured point. The 

 greater part of the absorption of liquid food is decidedly a 

 mere phvsical process. Supposing the food to be favour- 

 able and accfssible, 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 water with great energy, so that it will swell outi 

 and if the proportions are suitable, may even burst the bladder. 

 If the pre.isure is withstood there may be a filtration through 

 the bladder from the tension produced by the excess of absorp- 

 tion. Some experiments have been recently made by the 

 German physiologist, Hofmeister, showing that the endosrao- 

 818 is the principal cause of the flow of sap upwards into 

 plants. More than 150 years ago Hales showed that such 

 sap flowed out from plants, especially from the vine in springy 

 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 haa been confirmed by other observers ; and 

 Briicke has observed that the force depends 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 iuter- 

 commuuicating tubes, aud 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 themselves, and in 

 making some experiments on the common herbaceous garden 

 plants, he has found that the same force exists throughout all 

 of them, aud throughout all seasons, modified by conditions of 

 humidity 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 barometer tube, was fixed upon this ; it was found that the 

 force of the sap driven out from the crown of the root by 

 the absorption of water from the surrounding soil would 

 raise a column equal to 20 inches of mercury. 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 experiments are worthy 

 of credit ; the conditions 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, 

 when it was filtered out from the absorbing cells by the tension 

 produced by excessive absorption. lu these experiments a 

 most important difference in the pressure was found to result 

 from the amount of evaporation going on in the leaves above. 

 Where the evaporation was very rapid the pressure was re- 

 moved, and thus no tension or gorging of the vessels or soils 

 could result. This evaporation is important as connected 

 with the influence of excessive vegetation in modif^'ing the 

 condition of roots. It is well known — it is shown by the 

 experiments, I think, of most cultiva'ors — that excessive 

 development of the foliage is unfavourable to the develop- 

 ment to the roots of plants in the first instance. We have not 

 only to consider tlie dilTerent direction of the energy of the 

 plant, and the tendency of the growth of the foliage itself, but I 

 think the rapid evaporation from the leaves may cause too 



quick a flow of the sap through the atructuve, and prevent that 

 elaboration which is necessary from taking place in the roots. 

 I think all our observations hitherto tend to show that a cer- 

 tain amount of elaboration does take place in the roots them- 

 selves, aud if there is too rapid a flow of the nutriment 

 absorbed by the roots towards the upper part of the plant, 

 there is not a sufficient supply of elaborated nourishment in 

 the roots, and we get too crude a condition of sap in the upper 

 part. In the cases of plants cultivated for their flowers, we 

 see excessive luxuriance of foliage producing au indisposition 

 to flower and a throwing back of the whole growth. The same 

 thing occurs wherever we promote too great luxuriance of foli- 

 age in grain plants, as when we stimulate Wheat by excess of 

 nitrogenous manures. There is too great a development of 

 vegetative structure, and the same supply of climatal agency 

 (heat, light) is incapable of thoroughly ripening the seeds. 

 We see the same in garden plants, where too rich a soil pre- 

 vents the plants from floweriug. Almost any common weeds 

 if transplanted in a garden will run away and produce leaves, 

 aud perhaps flowers, but the seeds will scarcely ripen ; and 

 even with cultivated plants it is a common practice among 

 gardeners to prune the roots, to cut of the supplies of nourish- 

 ment, especially by withholding water, aud thus cause the 

 plant to turn upon itself as it were, and elaborate its food 

 rather than devote it to the production of new tissues. 



The absorption of food by the roots is concerned not only 

 with water, but with the most important of the materials out 

 of which vegetable structure is formed, namely, nitrogen 

 There is little doubt that the greater part of the nitrogen con- 

 tained in plants is absorbed by their roots. All experiments 

 tend in this direction. When I say that nitrogen is the most 

 important of the substances absorbed by plants, that is the 

 conclusion derived from both chemical and anatomical investi- 

 gations into the structure of plants. I may direct attention to 

 a paper I published in the last part of the Society's Journal 

 for evidence of the importanceof nitrogenous substances, cot only 

 in the development of the tissues, but in the formation of the 

 secretions or elaborated substances. Not only does the exami- 

 nation of the structures show this, but experiments with 

 manures also thoroughly prove it. That nitrogen in excess 

 will do mischief I have already stated, because it will 

 produce excessive growth and prevent proper elaboration ; but 

 a certain amount of nitrogen is necessary in order to convert 

 what may be called the wild plant into a tame one, to produce 

 that full developmeut of structure which we consider necessary 

 to the perfection of the plant. The cultivated plant has all 

 its favourable characters more fully developed than the wild 

 plant through this more abundant supply of food, aud the 

 most important of the stimuli in this food is undoubted'y 

 nitrogen. The recent experiments of M. Iloussiugault on the 

 growth of plants with or without nitrogen, and with aud 

 without phosphate of lime in combination, are very interesting 

 in this respect. He grew sunflowers — 1st, in a mixture 

 of pounded brick and sand, carefully purified ; 2nd, in the 

 same mixture with nitrate of potash aud ]ihosphate of lime 

 added ; and 3rd, in the same brick and sand mixture with car- 

 bonate of potash and phosphate of lime (thus giving the 

 alkali and keeping out the nitrogenous substances). The plants 

 grown in brick-dust and sand, after struggling through a few 

 weeks of vegetation, attained only a height of six inches, and 

 they produced a flower about an inch across ; and the same 

 result, with but little dltTerence, took place where the plants 

 were grown in the same mixture with phosphate of lime aud 

 carbonate of potash. The phosphate of lime did not enable 

 the plant to go beyond this starved condition ; and when the 

 whole was analyzed, the plcots were found to "ontain little 



