56 



Garden and Forest. 



[January 30, 1889. 



stem attains a height of about two feet, with large, hand- 

 some foliage, and enormous heads of yellow flowers, which 

 last a month or more. The plant is as easy to grow as 

 Paulow Ilia imperialis, but it is not quite as hardy. 

 Kew. W. Watson. 



A hardy shrub with beautiful foliage just now is the Andro- 

 meda Catesbai. Its stems are deep green, its leaves a dark 

 bronze, while its racemes of flower-buds, which are already 

 a half inch in length, in preparation for spring, are of a rosy 

 red color. Joseph Meehan 



Germantown. 



Principles of Physiological Botany, as Applied to 

 Horticulture and Forestry. 



y. — Some of the Effects of Evaporation from the Leaves on (i) 

 THE Plant, (2) the Soil, (3) the Atmosphere. 



ipFFECTS OF Evaporation from the Leaves on the 

 ■*--' Plant itself. — The amount of water which the plant 

 loses by means of the leaves is, as we have seen, relatively 

 large. Since, of course, it is only the water which escapes, 

 the mineral matters which the root-hairs brought in with the 

 water must remain behind. Therefore, the primary effect of 

 the evaporation from the leaves is concentration of dilute solu- 

 tions. The amount of mineral matter existing in leaves in 

 autumn is appreciably greater than in the early summer before 

 the work of concentration has proceeded far. The simplest 

 experiment to prove this is the combustion of a leaf plucked 

 from the tree in late spring or in early summer, and one taken 

 from the tree just before the close of the season. The latter 

 leaves a larger amount of ash, or incombustible matter, be- 

 hind, than the former. Concentration by evaporation is an 

 important factor in nutrition, as we shall see in a later paper. 



Effects OF Evaporation on the Soil.— The only question 

 to be considered with respect to this is, can the amount of 

 evaporation sensibly affect the amount of water in a saturated 

 soil 1 In other words, would it be possible to employ any 

 plants for drainage purposes ? The answer to this is not yet 

 quite clear, for the experiments upon the subject have not 

 been long enough continued, and in many instances they lack 

 scientific accuracy, not so much in the manner in which the 

 details have been stated, as in the disregard of disturbing con- 

 ditions. The most interesting instances are those of the 

 planting of Blue Gum, a species of Eucalyptus. This tree has 

 long possessed a certain reputation for its supposed power to 

 destroy, or at least to mitigate, the effects of miasm originating 

 in marshes. It should be stated at the outset that the leaves 

 yield a product known as eucalyptol, having more or less de- 

 cided antiseptic qualities. As the distinctive odor of this sub- 

 stance is given off by crushing or by bending the leaf, or even 

 by rapidly moving the leaf, especially on exposure to the sun's 

 rays, it has been believed by some writers that the effects in 

 reducing the deleterious influence of marsh-miasm may be 

 wholly attributed to the eucalyptol-like volatile matter. But 

 the plant has remarkable powers of giving off water in the 

 state of vapor. The amount has been variously stated by 

 different authors, but a fair average of the results of experi- 

 ments on Eucalyptus globulus would make it not far from two 

 quarts of water daily for every square yard of leaves. This 

 amount, though large, would be considered by some engi- 

 neers as too small to be a very efficient agent in the drainage 

 of swampy ground, but, as may be seen by the accounts of 

 success in the Roman Campagna,* especially at the "Three 

 Fountains," there are plainly two sides to this question. A 

 quotation from a work not readily accessible to the readers 

 of this journal will indicate that in the country of which the 

 species of Eucalyptus are natives, remarkable effects are at- 

 tributed to the odorous emanations from them. 



" That these enormous giants exercise an influence on the climate 

 of the district cannot be doubted, the whole atmosphere being impreg- 

 nated with a pleasant aromatic odour issuing from them. The odour 

 is especially noticeable in the early morning, and it produces an 

 agreeable feeling of invigoration, accompanied with a desire for exer- 

 cise. The inhabitants of the locality all enjoy excellent health, and 

 strict inquiries revealed the fact that no severe illness had occurred in 

 the Karri District within the memory of the oldest inhabitant, while 

 invalid visitors almost invariably improved in health. "^Rattrav and 

 Mill's " Forestry and Forest-Products." 1885 

 Prize Essay on Eucalyptus Plantations, p. 237 



Rattray and 

 From E. J. C. Brace's 



Other plants, notably the common Sunflower, have been 

 mentioned in connection with this matter of draining swampy 

 land, and good effects have been obtained where the task im- 



* Chamber^ yournal, io8, 193, 



posed upon the plants has not been wholly out of proportion 

 to the extent of foliage. In a detailed consideration of this 

 question, which the present paper cannot attempt, attention 

 should be directed (i) to the mechanical effect produced by 

 the larger roots, serving, as they do, about the same office as 

 the branches and twigs in an ordinary "brush drain, "and (2) to 

 the action of leafy branches in intercepting currents of air, 

 both of which have a bearing on the hygienic improvement 

 of wet land. 



Effects on the Air of Evaporation from the Leaves. — 

 Every one knows that the atmosphere possesses the power, 

 varying with the temperature, of taking up water and retain- 

 ing it in the form of vapor The total amount at any time is 

 its absolute humidity. 



When at any given time the atmosphere has taken up all 

 the moisture that can be retained at that temperature, it is said 

 to be saturated. Any proportional part of this amount ex- 

 pressed in percentage is its relative humidity. 



Observers have shown that the absolute humidity of the at- 

 mosphere in a forest is hardly greater than that of the air over 

 open ground. But, on the other hand, the relative humidity 

 is not far from six per cent, higher in the former than in the 

 latter case. Can a forest, by this slight difference in the 

 ainount of moisture which its atmosphere contains, or by the 

 innumerable points of contact presented by the leaves, or by 

 the cooling effect of evaporation from the leaves, affect the 

 precipitation of rain at any time .'' Or, to make the question 

 more general, do forests affect rainfall .'' This question, like 

 almost any about which not enough is known, has provoked 

 discussions which have been unnecessarily acrimonious. Even 

 the highest authorities are very far from agreement in this 

 matter. Ebermeyer, a distinguished and critical student of 

 the subject, wrote thus in 1873: 



" Forests increase the annual relative moisture of the air, but this 

 " influence is much more noticeable at high elevations than at low 

 "elevations. The precipitation of moisture (dew, cloud, rain, snow) 

 " takes place more readily on this account in wooded than in treeless 

 " regions, and the frequency and intensity of these precipitations in- 

 " crease with elevations above the surface of the sea. Moisture 

 " descends more readily and freely upon a wooded than upon a tree- 

 " less mountain of the same height. Forests affect rainfall only so far 

 " as they increase the relative amount of water held in the air, and 

 " thus bring the relative amount nearer the point of saturation ; thus, 

 "with fall of temperature in the forest, a part of the moisture is easily 

 "precipitated. . . Forests make the climate of a country moister, 

 " and especially so in the summer." 



Some later investigations in Bohemia, in European and 

 Asiatic Russia, and in India, point in the same direction, but 

 in our own country numerous comparisons may be fairly 

 interpreted in exactly the other way. The question waits for 

 further facts. 



The Pressure Exerted by Sap. — When a branch is cut in 

 one of the early days of spring there is generally a flow of 

 liquid froiTi the cut surface which varies in amount during 

 different periods of the twenty-four hours. An experimenter 

 in the last century showed that this flow from the cut surface 

 is not passive, but that the liquid is forced out, and he proved 

 by a pressure gauge that the amount of pressure was very great. 

 Similar experiments have been repeated both in Europe and 

 in this country with substantially the same results. Some of 

 the more interesting here were undertaken at the Massachu- 

 setts Agricultural College, in 1873, and showed that the pres- 

 sure might amount to even three atmospheres. The food 

 stored in the tissues of most of our forest trees during the sum- 

 mer is in the form of starch ; this is converted, toward the 

 end of winter, into some sort of sugar, thus passing into solu- 

 tion. This complex solution, together with the crude liquids 

 brought in through the roots froin the soil, constitutes the sap 

 as it flows from a wound. The factors determining the pres- 

 sure are (i) the osmotic force previously described and (2) the 

 expansion of the air in the air-spaces of the woody tissues. 

 At nightfall, or with a reduction of temperature, the pressure 

 ceases or becomes negative — that is, there is an actual suction 

 into the plant. 



There are one or two phenomena observed in the flow of 

 sap, which are as yet unexplained. 



Water-pores. — On the tips of the young leaves of some 

 plants, in warm mornings, drops of water can be seen, which 

 seem at first like drops of dew, but immediately on tiieir re- 

 moval other drops take their place. In some cases, particu- 

 larly in plants of the Calla Lily family, these drops follow one 

 another rapidly, and the water may even escape in the form of 

 a fine jet. The pores through which this water is forced are 

 somewhat like stomata, or breathing-pores, but they are not in 

 any sense true valves. 

 Cambridge, Mass. George Lincoln Goodale, 



