672 



NATURE 



\Oct, 24, 1878 



The value of e in each case above means the number 

 of 36oths of its period which the corresponding tidal con- 

 stituent has still to execute till its high-water from the 

 instant when the ideal star crosses the meridian of the 

 place. Thus if n denote the periodic speed of the par- 

 ticular tide in degrees per mean solar hour, its time of 



high-water is -, reckoned in mean solar hours after the 

 « 



transit of the ideal star. 



In this definition, and in the table of results, the fol- 

 lowing notation is employed ^ :— 



I to depote the mean inclination of the moon's orbit to the earth's 

 equator during the time of the series of tidal obser- 

 vations included in each instance. 



V „ „ the mean right ascension of the ascending node of 

 the moon's orbit on the earth's equator during the 

 same time. 



y ,, ,, the angular velocity of the earth's rotation. 



0- ,, ,, the mean angular velocity of the moon's revolution 

 round the earth. 



7) ,, „ the mean angular velocity of the earth round the sun. 



a ,, ,, the angular velocity of the progression of the moon's 

 perigee. 



" Speed " means the angular velocity of an arm revolving 

 uniformly in the period of any particular tidal constituent ; each 

 angular velocity being reckoned in degrees per mean solar hour. 



THE PHYSICAL FUNCTIONS OF LEA VES 



AN elaborate study on the above subject has lately 

 been published by Prof. J. Boussingault, of Paris, 

 in the Annales de Chimie et de Physique (vol. xiii. 

 pp. 289-394) ; in which the phenomena of absorption 

 and transpiration by leaves are treated at great length. 

 Since the memorable experiments of Hales in 1727, 

 recounted in his work on "Vegetable Statics," this 

 branch of vegetable physiology has been rarely touched, 

 and the carefully recorded observations of Boussingault. 

 carried out with the best of modern scientific appliances, 

 possess an unusual value. 



The first point studied was the loss of water by transpi- 

 ration from the leaves of plants under normal circum- 

 stances. For this purpose a healthy Jerusalem artichoke 

 {Helianthus ttiberosus) in a roomy flower-pot was chosen. 

 The top of the pot was covered with a sheet of india- 

 rubber, tightly inclosing the stem of the plant, and 

 provided with an opening for the admission of water. 

 The whole was then weighed, and the loss noted which 

 ensued under various circumstances, by evaporation of 

 water from the leaves, the plant receiving during the 

 experiment weighed normal amounts of water. The 

 total surface of the leaves of the plant (both upper and 

 lower sides) was carefully estimated, and the result 

 reckoned on the square metre. The averages of fourteen 

 experiments showed that the artichoke lost hourly, for 

 every square metre of foliage, the following amounts of 

 water : — in the sunshine sixty-five grammes, in the shade 

 eight grammes, during the night three grammes. 



In the next place the question was investigated whether 

 the absorption of water by plants, and the ascent of the 

 sap is due to the force resulting from the transpiration on 

 the surface of the leaves, or whether the roots exercise 

 also a certain amount of force to this end. For this 

 purpose experiments similar to the above were carried 

 out with various plants, firstly under normal circum- 

 stances, secondly with the stem minus the roots im- 

 mersed in water. As an instance we can take mint. 

 The plant with roots showed an hourly evaporation per 

 metre, of eighty-two grammes in the sunshine, and 

 thirty-six in the shade. Under the same condition, with- 

 out roots, the evaporation was sixteen and fifteen grammes 

 respectively. 



' .The values of I and i' are given to facilitate comparison with the equi- 

 librium values of the several tidal constituent":, according to Tables I. and 

 II. of the British Association Tidal Committee's Report of 1876. 



The results show that the absorption of water by plants 

 is determined in a great measure by the transpiration 

 occurring in the leaves, that this is maintained for a cer- 

 tain length of time without the assistance of the roots,, 

 but cannot continue long, being dependent on the injec- 

 tive power possessed by the roots. The effects of pres- 

 sure on the absorption was next examined, and it was 

 found possible by this means for a time in certain cases 

 to even more than replace the water lost by transpiration. 

 For example : a chestnut branch dipped in water was. 

 found to transpire hourly per metre of foliage, 1 6 grammes. 

 It was then inserted into a tube of water, and subjected 

 to the pressure of a column of water 2^ metres high. 

 Under these conditions the evaporation mounted to 55 

 grammes per hour, and the branch at the end of five 

 hours weighed more than at the commencement. 



The general result of these experiments shows the mutual 

 working of the various parts of the plant with reference 

 to the phenomena of transpiration. The roots absorbing 

 water from the soil by endosmose, direct it towards the 

 stem. Whether the motive force here is injection by the 

 roots or absorption resulting from the transpiration in 

 the green parts of the plant, or a union of both, is a 

 question still unsettled. The stem serves not only as a 

 passage for the water to reach the leaves, but also as a 

 reservoir to be drawn on during rapid evaporation. In 

 the leaves the sap is concentrated by the transpiration, 

 and the matters in solution enter into the cell formation,, 

 or, changed by the action of light, are distributed 

 throughout the plant by the descending sap. The cir-^ 

 culation would be quite similar to that in an animal,, 

 were it not for the irregularity. While the supply of 

 water from the roots varies but slightly, the loss by 

 evaporation from the leaves is subject to the greatest 

 fluctuations, according to the temperature and hygro- 

 scopic condition of the surrounding air. During these 

 periods the leaves draw on their stock of constitution 

 water and the supply in the stem ; and when both fail, 

 the phenomenon of wilting ensues. 



Numerous experiments were made on the difference ia 

 evaporation during the day attd during the night. Those 

 carried out with leaves of the grape vine gave the following 

 hourly averages per square metre of foliage ; in sunshine, 

 35 grammes ; in shade, 1 1 ; during the night,o-5. The trellis- 

 on which the vine was trained was i metre high and 38 

 metres long, and presented a surface of 138 square metres 

 of foliage. In sunny weather this would lose by evapora- 

 tion in the course of 24 hours, 48 kilogrammes of water, 

 and nearly half of that amount during cloudy weather. 

 To give an idea of the enormous amount of aqueous- 

 vapour dissipated by plants in the sunshine, calculation 

 showed that an acre of beets could lose in the course of 

 24 hours between 8,000 and 9,000 kilogrammes. Another 

 experiment made with a chestnut-tree 35?years old showed 

 that it lost over 60 litres of water in the course of 24 

 hours. The structure of the leaf, however, containing 

 70-80 per cent, of water, and possessing a thickness fre- 

 quently of but i^jth of a millimetre, would suggest the 

 question why the evaporation is not much more rapid. 

 The answer to this is found in the peculiar structure of 

 the tissue forming the epidermis, designed especially ta 

 moderate the transpiration. In order to see the remark- 

 able retentive power exercised by this epidermis, one can. 

 expose for a few hours to the sun two cactus leaves of the 

 same superficies, one of which has been deprived of 

 its epidermis. The evaporation in the latter case will be 

 about fifteen times as rapid as in the other. It is the 

 presence of a similar tissue forming the skin of fruits 

 which prevents an otherwise rapid evaporation. For 

 instance, an apple deprived of its skin loses 55 times as 

 much water as a whole specimen in the same time. 

 Losses by rapid evaporation lessen notably the physiolo- 

 gical energy of leaves. Thus an oleander leaf containing 

 60 per cent, of water, when introduced into an atmo- 



