April 13, 1888.] 



SCIENTIFIC NEW^S. 



353 



THE FORMATION AND FUNCTIONS OF 

 STOMATES.— II. 



(JConcluded from page 309.) 



"DEHIND the stomal orifice there always exists, in the 

 ^ tissue of the leaf, a hollow space several times larger 

 than the stoma, known as a respiratory cavity ; by means 

 of this space the work of transpiration or evaporation of 

 water is greatly facilitated. Into these spaces vapour 

 from the water in the surrounding tissue of the leaf is 

 given oft", and passes out at the stoma when open, dry 

 air diffusing in to take its place. There the influence 

 of sunlight again comes into play, the accompanying 

 heat first increasing the watery vapour, then causing, by 

 expansion, its more vigorous expulsion from the res- 

 piratory cavity. But as the internal tissue of the leaf 

 loses water, it draws a fresh supply in the form of 

 sap from the veins, which consist of very fine tubes and 

 woody fibres, and these in turn by their connection with 

 similar tubes in the leaf-stalks, branches, stem and roots, 

 draw up water impregnated with inorganic salts from 

 the soil. 



Transpiration or the elimination of water, however, 

 though the chief, is not the onlj', function of the 

 Stomates. It has been mentioned how the sunlight, by 

 its influence on the grains of chlorophyll, or green 

 matter in the guard-cells, brings about the formation of 

 starch out of the carbonic acid gas in the air and the 

 water in the leaf — starch being composed of carbon, 

 hydrogen, and oxygen. But this process, though a use- 

 ful one to the guard-cells, produces too small an amount 

 of starch to be of any importance in the general nutrition 

 of the plant. It is in the mass of chloropyll-bearing 

 cells which underlies the transparent epidermis, and 

 gives to the foliage of plants its refreshing and character- 

 istic colour, that the general supply of starch is produced. 

 But before starch can be made it is necessary that 

 carbonic acid gas enter the leaf, and come into contact 

 with the green cells. And here again the advantage of the 

 stomal communication comes into requisition, for although 

 carbonic acid, being a gas, can penetrate the thin mem- 

 brane of the epidermis, it cannot do so as rapidly as 

 through the open stomates. The carbonic acid gas, upon 

 contact with the green-cells, is split up into its two com- 

 ponent elements, carbon and oxygen, the carbon being 

 retained and the oxygen being given oft' at the open 

 stomate. This is the process which renders the presence 

 of trees in moderate numbers so healthful in thickly- 

 populated districts. It is not, however, as it is sometimes 

 so misunderstood, a process of respiration but of assimila- 

 iioit. Consider then on a summer's morning what a 

 wonderful amount of silent activity is gradually set in 

 motion in the plant world by the Isreaking forth of sun- 

 light upon the earth. In the trees, with their milions of 

 stomates (the lilac leaf has 160,000 per square inch of 

 under-surface), the watery vapour locked up in the 

 respiratory cavities passes out between the dilating 

 guard-cells into the air ; the sap slowly moves upwards 

 to supply the fresh demand for vapour, bringing with it 

 earth-salts from the soil to the leaf-laboratories, there to 

 be compounded into plant food with the starch which is 

 being formed out of the incoming carbonic acid gas, 

 whilst oxygen passes out into the air to carry on the 

 respiration necessary to all life whether animal or 

 vegetable. 



Transpiration being the chief work of the Stomates, it 

 may not be uninteresting to consider some of the ways 



in which it affects plants, and how it in turn is affected 

 by internal and external influences. Such considerations 

 must, however, be limited to land-plants, as stomates 

 are seldom present on submerged water-plants, and 

 only those land-plants which exhibit a distinction into 

 leaf and stem. Colourless parasitical plants also may be 

 excluded ; an instance of the scarcity of stomates on 

 these is seen in the mistletoe, which possesses only 200 

 to the square inch, a very insignificant number 

 compared with that in the lilac, cited above. To only 

 such land-plants then as are not lower than the mosses 

 and liverworts, and are not parasites destitute of 

 chlorophyll, will the following remarks apply. 



The existence of transpiration may be easily proved. 

 When a plant is plucked and left in the sunlight it 

 withers through loss of moisture, and this loss may 

 be tested by comparing the weight of the plant at the 

 time of plucking, and some hours afterwards. This is 

 the reason why plants so quickly become flaccid and 

 lifeless when taken from the soil, and not, as it is some- 

 times imagined, from want of food. This also is the 

 reason why leaves and stems droop in strong sunlight, 

 and freshen at sundown ; the waning of the sun's heat 

 and light renewing the swelling of their cells. The 

 greater part of this evaporation must take place through 

 the stomates, as the skin of plants is specially formed to 

 resist the passage of aqueous vapour. Again, those who 

 keep ferns under shades know how bedewed with mois- 

 ture the inside of the glass covering becomes, and the 

 amount of water thus transpired may be readily ascer- 

 tained by putting a quantity of dry chloride of lime or 

 concentrated sulphuric acid into an open dish, and then 

 placing it under the shade with the fern. These sub- 

 stances absorb moisture very readily, and the difference 

 between their weight when put under the shade, and 

 when taken out, will give about the weight of water 

 transpired during the time. To make the experiment 

 more conclusive a pot of earth, subject to equal con- 

 ditions of dampness and sunlight, may be tested in a 

 similar manner. The quantity of water thus given off 

 by plants is something considerable, and may be appre- 

 ciated from the following details : — A healthy tobacco 

 plant, or a sunflower the height of a man, will transpire 

 from a pint and a half to a quart on a summer's day, 

 and full-grown oaks, poplars, and other large trees from 

 eleven to twenty-two gallons daily, and Hohnel has 

 computed that a patch of a little over two acres of beech 

 forest a hundred and fifteen years old yields about 

 500,000 to 700,000 gallons from June ist to December 

 ist. Great humidity is thus produced in densely- 

 wooded districts, and the eftects of transpiration meteoro- 

 logically is such that the gradual removal of a large 

 tract of forest has considerably decreased the rainfall in 

 the neighbourhood. It is for this reason that the pre- 

 sence of large quantities of trees is apt to render the atmo- 

 sphere of a place so damp as to be unhealthy, whilst on 

 the other hand the great scarcity of trees produces the 

 same effect through too much dryness, and the non- 

 absorption of carbonic acid gas ; it is in the medium 

 that salubrity lies. 



(7b be continued.) 



The Cold Season. — According to the Popular Science 

 Monthly, recent winters in Iowa have been more severe 

 than earlier ones. In January last the thermometer was 

 at or below 0° F., every night from the i ith to the 20th. 



