1844.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



1^5 



called the medullary rays, forming in limber the silver grain: The 

 branches are but extensions of the stem, the smallest fibre resembhng the 

 wood of the stem. So, also, are the roots ; but as Ihey taper off the pith dis- 

 appears, and at length the wood and bark, only cellular matter being at the 

 extreme points ; these are the spongioles, and in these the power of absorption 

 resides ; it is on account of the injury inevitably done to these that trans- 

 planting is so dangerous an operation. These absorb the water in the soil 

 holding saline substances and carbonic acid in solution. The question has 

 been raised, can the roots of plants select their food ? and although there have 

 been many experimenters, the evidence is very contradictory. Dr. Daubeny 

 made some experiments on barley and peas with a solution of slrontia, and 

 found that they would not take it up. Saussure obtained similar results with 

 acetate of lime and common salt. But their selective power is at al! events 

 limited, as they have been made to absorb solutions of arsenic, corrosive sub- 

 limate, opium, and tobacco, and their effects are said somewhat to resemble 

 those on animals. The excretory function of the root was once supposed to 

 be the reason of the necessity of a rotation of ^crops, as a plant was said to 

 throw out substances which were poisonous to plants of the same kind. 

 Now, however, it is explained by saying that each plant requires particular 

 inorganic food, and if one crop has removed al! of that kind from the soil, of 

 course a similar crop could not thrive. Thus turnips will grow after wheat, 

 because, as they require diflcrent constituents, the wheat has not removed 

 those which the turnip requires. 



The power of the root to absorb liquids from the soil is generally attributed 

 to two actions, endosmose and capillary attraction. Endosmose, or the pro- 

 perty of thin fluids to pass through porous bodies if thicker fluids are on the 

 other side, is well illustrated by immersing the ends of three tubes tied round 

 with membrane into water, having previously poured into them solutions of 

 gum, sugar, and some alcohol. Tlie water passing through will cause the 

 level of the thicker fluids to be consiilerably raised. The capillary action of 

 porous bodies is seen when the end of a cane is dipped into spirits of turpen- 

 tine ; it rises up through the length and may be lighted at the other end. 

 The sap hi a plant decidedly circulates, and its course has been distinctly 

 traced. It ascends from the root by the outer layer of wood to the leaves, 

 there, being spread over a large surface gives up a great quantity of water, is 

 acted on by light, and flowing down the inner bark, much thickened, deposits 

 new wood in its course. Thus each year a ring of new wood and new bark is 

 formed, and by counting the rings of wood;in the section of a tree, its age 

 may be ascertained. 



LeCTDRE X. AND LAST. 



The direction of circulation of the sap when it forms new wood, may be ren- 

 dered evident by ringing the bark of a tree ; it will be found that the new 

 wood will always be deposited from the upper part of the cut, showing that 

 in the bark it is travelling downwards. The force with which the sap rises is 

 very great, and experiments have been made by cutting a branch in the 

 spring time, and so confining it in a tube with mercury that the force with 

 which it rises can be measured. An application of this has been recently made 

 to the preserving timber by means of impregnating it with various saline 

 fluids. The fresh cut stem is immersed in the fluid, with the leaves and 

 branches still attached ; evaporation going on at the surface of the leaf, the 

 fluid is drawn into every part much more thoroughly than can be done by 

 other means. The circulation is never stopped in a tree, for though it is 

 much less in winter, it still goes on. A thermometer placed in the interior of 

 a tree shows it to be a little warmer than the air; this Is easily accounted 

 for ; the earth being warmer than the air, the sap drawn from it must neces- 

 sarily be so also. This is e-tceedingly useful, as it prevents it from freezing 

 and cracking its delicate vessels. ' 



The leaf, which is a flat expansion of the wood and bark, contains vessels 

 which bring the sap in, and others which carry it out and down into the 

 inner bark. Few have an idea at the enormous transpiration continually 

 taking place from the surface of leaves. Dr. Hales found that a sunflower 

 exhaled from 20 to 30oz. of water in a warm day, and 5 oz. at night. In 

 dewy or damp weather it did not exhale any. By this means the sap, which 

 enters it watery and insipid, goes out from it much thicker and sapid. This 

 also is the reason why leaves yield so much more ash than any other part. 

 The organs of the leaves are very delicate, and few can bear the soots, ashes, 

 acids, he, which load the air of London. Some, however, can, and a list of 

 them has been published by Mr. M'CuUock. Green-house plants are delicate, 

 and sometimes fall victims to injurious vapours diflused through the house. 

 At Godsden, some pipes had been covered with a paint containing coal tar ; 

 almost every plant was injured or killed. This proves that plants can absorb 

 vapours by their leaves, as they show the effects before it has time to get to 

 the roots. The mischief that cold easterly winds cause to the leaves is as 

 great as though they were placed before a fire. 



These are merely the mechanical powers of the leaf; the chemical are 

 powerful and astonishing. They can be explained, but difficultly imitated. 

 The chemist, for instance, can readily make carbon into carbonic acid, but he 

 can scarcely get the carbon out again ; this the plant is always doing, evolving 



the oxygen. This is the general result, though at timesit Is slightly different, 

 when, for instance, the sap is to be acid, the oxygen must be retained. When 

 it is to be resinous, as in fir trees, excess of hydrogen is wanted ; deriving 

 this principally from water, oxygen must be liberated from this source, as 

 well as during the formation of wood. The rapidity wilh which the leaves of 

 a tree deprive the air of its carbonic acid is no chemical fiction, as there are 

 abundance of experiments on record to prove it. Boussingault brought the 

 limb of a vine into a jar, and found that it deprived the air of carbonic acid 

 and evolved oxygen as fast as he carried a tolerably rapid current through it. 

 For this It requires a bright sun, and that is why plants grow so quickly in a 

 tropical climate. Decandolle immersed aquatic plants in water impregnated 

 with carbonic acid, and found that they deprived the water of the gas, re- 

 placing it with oxygen. Thus they perform for fishes what land plants do 

 for land animals. Dr. Gilly made a mixture of gases having a great excess of 

 carbonic acid, and immersed a turf of grass therein ; after four hours' expo- 

 sure to sunshine, he got the following result : — 



Before After 



Nitrogen . . 105 105 



Carbonic Acid .57 4 



Oxygen . . 28 78 



190 187 



With the exception of a little loss of oxygen, the whole of the carbonic acid 

 absorbed was replaced by that gas. Saussure obtained the same result from 

 Lythrum salicana, and Vinca minor. Priestley, long ago, placed sprigs of 

 mint, whilst growing, In air which had been destroyed by a mouse or a 

 candle, and found that it restored it to a proper state. It was then supposed 

 to be due to the mint sending out an agreeable odour ; but he proved that 

 plants w ith a disagreeable odour would do the same. In his correspondence 

 with Franklin, given in the Philosophical Magazine, he says that he hopes 

 the knowledge of this will prevent persons from cutting trees growing near 

 houses. Other remarkable changes can be proved to be effected by plants. 

 The little water cress, as well as many other plants, has the power of decom- 

 posing sulphuric acid, retaining the sulphur, and liberating the oxygen ; 

 other plants, in like manner, decompose phosphoric acid. 



Dumas has drawn up a very ingenious contrast between animals and vege- 

 tables. 



Vegetables. 



'D.».i.,«» f Azotised principles. 

 Produce |yj,^^^,,_ J^^^^ J,j^_ ^^_ 



J Carbonic acid 

 Decompose < Water 



vAmmonia 

 Evolve Oxygen 



Absorb Heat 



Are an apparatus of de-oxidizement. 

 Are stationary. 



Animals. 

 f Azotised principles. 

 \.Slarch, suyar, oils, 8ic. 



{Carbonic acid 

 Water 

 Amirionia 

 Oxygen 

 Heat 



Are an apparatus of oxidlzement. 

 Are locomotive. 



Consume 



Produce 



Absorb 

 Evolve 



These are not true of the petals, &c., of blossoms, which act like animals, 

 sometimes evolving ammonia, always absorbing oxygeu, and occasionally 

 producing very great heat. 



When the bark of a plant Is green, it acts like the leaf, therefore in herba- 

 ceous plants the whole surface acts alike. In ihe cactus there are no true 

 leaves, the stem supplying its place. The bark of the root, in sume cases, 

 secretes a substance not found elsewhere. Phloridzin is thus found beneath 

 the bark of the root of apple and pear trees. 



The circumstances which influence and modify the growth of plants, are 

 chemical and mechanical. At the head of the chemical must be placed light. 

 By analysing rays cl light it has been found that the violet or chemical rays 

 are those in which the power of light on plants resides. Mr. E. Solly, of the 

 Horticultural Gardens, has made experiments on the action of various co- 

 loured glasses on the light for plants, and he has obtained very excellent 

 results by the use of violet glass, that is, by cutting off' all the rays of light 

 but the violet. The influence of mineral manures has been shown to be two- 

 fold, by passing into the roots, and also by yielding parts of themselves, as in 

 the case of the nitrates. Phosphorus is indispensable for wheat, silica for 

 grass. Lime acts on crops by inducing changes in the organic matter, by 

 neutralizing acids, by decomposing inorganic salts, and by liberating ammo- 

 nia from its combinations, as, for instance, from rain water. In tracing the 

 growth of vegetables, their power of absorbing, changing, and recombining, 

 the materials obtained from the soil and the air, has been seen, as also their 

 influence when they are returned to the soil, acting as manure, loosening the 

 soil, and returning to it its inorganic salts. The parts of the plant thus used 

 for manure has great influence ; thus the leaves arc much better than saw- 

 dust. There are also advaniages attending ploughing It in in a green state ; 

 this is frequently dune by agriculturists, as, after fermenting, it yields a rich 

 mould. For this reason it is judicious to gather together the hedge-row 

 weeds, and bury them in the soil. Vineyards can be manured by their own 

 cuttings, and for a length of years will scarcely require any other mannre. 

 This method has been revived of late, but It was a practice resorted to by the 

 Romans. The same principle will apply to most crops, as by green crop 

 ploughiug both the organic and inorganic cuustitueuls are returned. 



