356 



THE FARMERS' REGISTER. 



light. The last experiment shows that, while 

 the sunflower was losing from tweniy to thirty 

 ounces of water daily during the day, it lost only 

 three ounces during the night without dew, and 

 that there was n'o loss whatever if a slight dew 

 were present. Here it is probable that the small 

 amount which was lost at night was parted with 

 by the sides o( the garden pot, and that the plant 

 itself lost nothing, lor it is in evidence that the 

 perspiration of plants is in proportion to the quan- 

 tity of sunlight that strikes them, and that in 

 darkness they perspire little or not at all.* it is no 

 doubt true, that in a dry atmosphere plants will 

 lose their water day and night ; but it is equally 

 certain that under such circumstances they will 

 lose very much more by day thah by night. They 

 will, however, lose much more by day in a dry 

 atmosphere in a given time, than they will in an 

 atmosphere abounding in moisture. 



71. Although perspiration thus appears to be 

 principally excited by the solar rays, and to be in 

 a given plant in proportion to their intensity, yet 

 we'are not authorized in concluding that perspir- 

 ation is not increased or diminished t)y the medium 

 in which a plant grows. Immersed in water, 

 perspiration is necessarily arrested ; in an ordi- 

 nary atmosphere, it will be in proportion to the 

 quantity of elastic vapor the atmosphere may 

 contain ; and it is probable, although there are 

 no experiments upon the subject, that it is in- 

 creased in proportion to the rarefaction of the air. 



72. Since a plant does not perspire at night, 

 and since its absorbing points, the roots, remain 

 during that period in contact with the same 

 humid medium as durintr the day, they will attract 

 fluid into the system of the plant during the night, 

 and. consequently, the weight of the individual 

 willbe increased, as Hales lounJ to be the case. 

 Jn like manner, if plants in the shade are abun- 

 dantly supplied with moisiureat the roots, they 

 also will gain more than they can lose; and, as 

 this will be a constant action, the result must 

 necessarily be to render all their parts soft and 

 watery. 



73. It is evident, from what has bpen staled, 

 that Isaves must derive the food they digest from 

 the earth through the medium of the roots; and 

 that they, while alive, maintain a kind of perpe-. 

 taal sucking action upon the stem, which is com- 

 municated To the spon(releis. That this must be 

 of a very powerful nature is apparent from the 

 fact, that the smallest leaf at the extremity of !he 

 branch of a lofty tree must assist in setting in 

 action the absorbing power of roots, at a distance 

 eqaal, perhaps, to three thousand times its own 

 length. If this reciprocal action is not main- 

 tained w't'iout interruption, and if any th ng occurs 

 to check it during the period of vegetation, the 

 plant will suffer in proportion to the amount of 

 interruption. For example, if the roots are placed 

 in a warmer medium than the branches, and 

 are thus induced to absorb fluid faster than the 

 slower action of the leaves can consume it, the 

 superfluous sap will burst through the stem and 

 distend its tissue till the excitability is impaired 



* M. De CandoUe distiuguishes between exhalaison 

 or perspiration, which is a vital action, deperdilion or 

 evaporation, which is merely physical. But the latter 

 is too small in amount to be worth taking into account 

 i'or practical purposes. 



or destroyed. Or if, on the other hand, a branch 

 is caused to grow in a warm medium, while the 

 roots remain in a very cold medium, the former 

 will consume the liquid sap faster than the latter 

 can supply it, and the consequence will be, that 

 the leaves will die, or the fruit will fall offj or the 

 flowers be unable to set their fruit, from want of 

 a constant and suflicient supply of food. Not 

 that it is necessary for the temperature of the earth 

 and air to be equal, lor this does not happen in 

 nature; but it is requisite that they should have 

 some near relation to each other. 



74. It is generally, however, believed, that 

 leaves absorb fluid from the air ; and their siomates 

 appear well adapted for that purpose, by their 

 position in most abundance on the under side of 

 leaves ; and the possibility of recovering drooping 

 or sickly plants, by syringing their epidermis 

 copiously, seems to render this fact almost cer- 

 tain.* It is, however, thought by some, that 

 leaves have no power of absorbing water, even 

 in an elastic state ; and that tlie renovation of 

 plants by syringing is owing to a diminution of 

 perspiration. 



75. It is to the action of leaves, — to the decom- 

 position of their carbonic acid, and of their water ; 

 to the separation of the aqueous particles of the 

 sap from the solid parts that were dissolved in it ; 

 to the deposition thus efiiected of various earthy 

 and other substances, either introduced into plants, 

 as silex and metallic salts, or formed there, as 

 the vegetable alkaloids; to the extrication of 

 nitrogen; and, probably, to other causes as yet 

 unknown, — that the formation of the peculiar 

 secretions of plants, ol whatever kind, is owing. 

 And this is hrouiiht about principally, if not ex- 

 clusively, by the agency of light. Their green 

 color becomes intense, in proportion to their expo- 



* Mr. Knight entertained the opinion, that water is 

 sometimes absorbed by leaves to such an extent as to 

 cause a descent of the sap through the alburnum ; a 

 derangement of lunction to which he even ascribed 

 the attacks of mildew fungi upon plants. The second- 

 ary and immediate causes, lie says, of this disease, 

 and of its congeners, " have long appeared to me to 

 be the want of a sufficient supply of moisture from 

 the soil, with excess of humidity in the air, particu- 

 larly if the plants be exposed to a temperature below 

 that to which they have been accustomed. If damp 

 and cold weather in July succeed that which has 

 been warm and bright, without the intervention of 

 sufficient rain to moisten the ground to some depth, 

 the wheat crop is generally much injured by mildew. 

 I suspect that in such cases an injurious absorption 

 of moisture, by the leaves and stems of the wheat 

 plants, takes place : and I have proved that under 

 similar circumstances much water will be absorbed 

 by the leaves of trees, and carried downwards through 

 their alburnous substance ; though it is certainly 

 through this substance that the sap rises, under othtr 

 circumstances. If a branch be taken from a tree 

 when its leaves are mature, and one leaf be kept con- 

 stantly wet, that leaf will absorb moisture, and supply 

 another leaf below it upon the branch, even .though 

 all communication between them through the bark 

 be intersected ; and, if a similar absorption takes place 

 in the straws of wheat, or the stems of other plants, 

 and a retrograde motion of the fluids be produced, I 

 conceive that the ascent of the true sap or organizable 

 matter into the seed-vessels must be retarded, and that 

 it may become the food of the parasitical plants, 

 which then only may grow luxuriant and injurious." 

 (Hart. Tram., i. 86.) 



