SAP 



SAP. 



Ml 



prin,-. Then are but few observations upon the quantity of water 

 needed by plant*, but the fact* supplied by Hales and Schiiblor show 

 that rain, after making allowances for that which flows away and is 

 evaporated, does not supply more than a tenth part of what is neces- 

 sary. It in unaccountable and inexcusable that not a single botanist, 

 inoe the time of Hales, should have taken up and carried on his 

 experiment'. If we take the previous calculation* of the quantity 

 of water required by plant* in England, which is deduced from Halos's 

 experiment!., and which agree with those of Schiibler on Poa anmta, 

 we shall obtain the following approximative results : According to 

 SchnbliT, there falls in Kngland upon the acre of 40,000 square feet, 

 at the utmost 1,600,000 Ibs. of water during 120 days of summer. 

 According to the researches of Dalton, Miiller, Berghau?, and Dausse, 

 '. least a third part of this water flows away into the rivers, but it 

 i* probably more than thin, as the great rapidity of the flow of the 

 w it or in steep, hilly, and mountainous regions is not sufficiently taken 

 i.ito consideration. A considerable but not accurately to bo esti- 

 mated quantity of water evaporates immediately after the fall of rain, 

 ai the vapourous state of the atmosphere indicates. From this it 

 wjuld appear that at the most there i left disposable for plants and 

 future evaporation 800,000 Ibs. of water on the acre. Now this 

 quantity of water, according to the preceding experiments, would not 

 c iver more than two-thirds of the demand of an acre of cabbages, 

 half of the demand of an acre of sunflowers, or of the Jerusalem 

 Artichoke (Ifeliantkut tnberonu), the fourth of a fruit garden, the 

 fifth of a hop garden, and about the seventh or eighth of a meadow. 

 It must be recollected that here only the water is taken into ca'.cu- 

 la'ion which is given out from the plants and weeds growing in a 

 m -:i iow, but that which is afforded by the evaporation of the soil 

 iUelf cannot amount to less than 2 000.000 Iba. for an acre. Thus 

 much is very evident from these calculations, that the quantity of 

 rain that falls upon the earth is no more a measure of the quantity 

 needed or consumed by the plant, than is the quantity of humus, an 

 index to the fertility of the soil. We may learn from this that the 

 quantity of rain which falls in a given region is not a measure of its 

 fruitfulness, but the quantity of moisture, the absolute and relative 

 quantity of vapour, which yearly, and especially during those months 

 which are most important for vegetation, is contained in the atmos- 

 phere." (Schleiden, ' Principles of Scientific Botany.') 



Carbonic acid gas supplies plants with the carbon of their tissues. 

 At one time it was supposed that the carbon of plants was supplied 

 directly from the soil, by the agency of a substance called Humus. 

 There are many facts in nature which show that this is impossible, 

 for there are districts of the earth's surface which are constantly 

 yielding carbon in the vegetable and animal products which they 

 supply, and to which no humus is added to keep up the loss of carbon. 

 This U the case with the produce of Switzerland, which for centuries 

 ha* been sent into Europe, and yet no system of manuring is carried 

 on in the agriculture of the Alps. Many districts of England and 

 France are now covered with a fertile vegetation which a few centuries 

 *fo were barren sands. Such facts prove that the supply of carbonic 

 acid is from the air and not from the soil The sources of carbonic 

 acid gas on the surface of the earth are very numerous and constantly 

 operative, and yet this gas does not accumulate in the atmosphere. 

 The only conclusion that can be come to with regard to its disappear- 

 ance in, that it has been taken up by plants. It has also been proved 

 by direct experiment that plants will not live without a supply of 

 carbonic acid gat, and that they flourish where this gas is present. 



The following are the sources of this gas : 



1. The respiration of man and animals. In one year it is calculated 

 that a single human being throws 225 Ibs. of carbon into the atmos- 

 phere. If this is multiplied by 1,000,000,000, it will give the enor- 

 mout amount of carbon thrown into the atmosphere every year by 

 man alone, which must at least bo doubled for the rest of the animal 

 creation. [KapiEATiox.] 



2. Combustion is a vast source of carbonic acid gas. Man needs 

 beat to maintain his own temperature, to cook his food, and to manu- 

 facture and move about with. The quantities of carbonic acid thus 

 thrown into the atmosphere must be even greater than that produced 

 by respiration, and at the present day must constitute a very prominent 

 source of supply of food to the vegetable kingdom. 



3. All sugar in decomposing ferment*, and a large natural supply o 

 carbonic acid is probably thus afforded, but in the manufacture o 

 alcohol by man, the quantity of carbonic acid thus formed most be 

 very considerable. In all vegetable and animal decay carbonic acic 

 is formed, and thus animals and plants are converted into an inorganic 

 MibsUnce before their elements are again used in the nutrition o" 

 organic beings. 



4. Another source of carbonic acid is the interior of the earth. In 

 all volcanic districts the springs come up charged with carbonic aci< 

 gal, and this gas is found constantly present in lake, river, and sprin) 

 water. It is sUo found that the gases which are discharged from 

 active volcanoes consist principally of carbonic acid gas. This accounts 

 f>r the destructive effects of the vapours of a volcano, where person 

 an exposed to their direct action. This purely mineral source o 

 carbonic acid U interesting, as proving that a constant supply of 

 carbon is kept up, not only for sustaining, but for allowing an increase 

 in the vegetable and animal kingdoms from the mineral kingdom alone. 



Unless this source was known, it woulJ be difficult to explain from 

 whence the original supplies of carbon came, as we have every reason 

 to believe that plants and animals have been created as individuals or 

 pairs. To take the human race alone there must be 500,000,000 times 

 a much carbon existing in it at the present moment as existed in the 

 jodiea of our first parents. Whence the source of this supply, if not 

 rom the mineral kingdom ? 



The sources of the supply of ammonia are the same as those of 

 arbonic acid. The excretions of animals, the combustion of wood 

 ud coal, the decomposition of nitrogenous matters, the gases of vol- 

 sanoes, ore all sources of this gas. 



The constituents of the sap which are not necessary for the growth 

 if all plants are principally the metallic oxides, which it is well known 

 enter very largely into the composition of some plants. The most 

 common of these are the oxides of potassium, sodium, calcium, an<l 

 magnesium. These occur in combination with various acids, but the 

 acid is not found to exercise so much influence on the plant as the 

 >ase. Although any of these .oxides when presented in solution would 

 > absorbed by plants, it would be only those adapted to the peculiar 

 iabit of the plant that would be appropriated. Thus plants which 

 ;row naturally on the sea-shora, and require soda for their growth, 

 ill take up potassa when presented to them in combination with soda, 

 nit they would reject the potassa by excretion and retain the soda, 

 ROOT.] The sap therefore which is found in plants varies in com- 

 position both from the nature of the soil and the nature of the plant. 



From the soil the sap is conveyed by the roots into the plant, and 

 is not long before it undergoes certain changes in its composition, 

 }ut the nature of these change?, and the period at which they take 

 place, are not well known. It is however a fact that the nearer a 

 ;ree is tapped to its root, the more fluid is the sap which exudes. 

 The channels through which the sap parses in its upward course are 

 also a subject of difficulty. Various observers have contended for 

 each of the different tissues being the sole conveyor of this fluid, but 

 it is most probable that, with the exception of the spiral vessels, 

 which seem appropriated to the conveyance of air, all the tissues of 

 a plant ore engaged in conveying sap. There are some parts which 

 seem to convey more than others, and the younger tissues are always 

 more filled with fluid than the older. Thus, whon the trunk of a tree 

 is cut through in spring, sap will be seen to exude from all part* of 

 the cut surface, but in greatest quantities from the alburnum or sap- 

 wood, the most recently formed portion of the timber. 



By whatever channels the sap pursues its upward course, we find 

 that it undergoes great changes between the period of its absorption 

 from the soil and its ultimate disposition in the secretions of the 

 plant. The most important of these changes is the loss of a largo 

 portion of that water which it possessed when first absorbed. This 

 water is got rid of by the process of exhalation, which takes place 

 under the influence of external heat and light In this way it baa 

 been ascertained that a common sunflower, three feet in height, will 

 lose one pound four ounces of water every day ; and a common cabbage 

 one pound three ounce*. Hales contrived to measure the force with 

 which plants exhaled during the summer, and computed that in some 

 plants it was five times as great as that which impels the blood in the 

 crural artery of a horse. The part of the plant in which this process 

 goes on most rapidly is the leaf, which, from its extensive surface and 

 delicate structure, is well adapted for the performance of the function. 

 For this purpose however the leaf is endowed with especial organ* 

 called stomatea. The stomates are small openings in the cuticle of 

 the leaf, tbo number of which varies exceedingly in different leaves, 

 and the process of exhalation bear.) a direct proportion to their 

 number. Exhalation goes on principally during the day, under the 

 influence of the light of the sun, and almost completely ceases when 

 the aim's rays are withdrawn. It is on this account that plants loose 

 BO rapidly their freshness on exposure to the light of the sun, when 

 they have been plucked, or otherwise deprived of the means of obtaining 

 a fresh supply of water. When the sap has arrived at the leaf and 

 lost a certain quantity of water, it has been supposed that the peculiar 

 secretions of the plant are there formed and sent down into the plant 

 This has been called the 'descent of the sap.' It is somewhat 

 difficult to imagine how these secretions can pass bock through so 

 delicate an organ as the petiole, and pass down through bark or stem, 

 even into the roots of the plant. The argument* which have been 

 used for adopting this view are the facts that the peculiar secretions 

 of the plant are not produced till the leaves are developed ; that in 

 ringing a tree the largest quantities of secretion is found in the upper 

 and not the lower part of the wound, and that when the leaves are 

 injured no secretions are found. It is answered that all the secretions 

 of plants are formed in the interior of the cells where they are found, 

 and that their formation depends on the leaves, not because formed 

 in the leaves, but because the leaves by their function of exhalation 

 produce a supply of nutritive materials, which is carried to every part 

 of the plant, and thus there could be no secretions without the leaves. 

 Again, when a plant is ringed or cut, the amount of effusion of sap or 

 juice* will depend on physical circumstance*, and a greater supply of 

 secreted matter is found in the upper part of a ring, because there is 

 most liquid matter to be effused from the trunk and branches of the 

 tree above, and because of it* dependent position. 



The cause of the progression of the cap in plants has been a fruitful 



