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HISTORY OF THE VEGETABLE KINGDOM. 



tioii is an exudation of sap, too gross or too 

 abundant to be dissipated immediately, and 

 wliicli lienoe acuniulates on tlie surface of the 

 leaf. It is very generally to be met with in the 

 course of the summer on the leaves of the maple, 

 poplar, and lime tree ; but particularly on the 

 surface exposed to the sun, which it sometimes 

 wholly covers. Its physical as well as chemical 

 qualities are very different in diiferent species of 

 plants, so that it is not always merely an exuda- 

 tion of sap, but of sap in a high state of elabora- 

 tion, or mingled with the peculiar juices or 

 secretions of the plant. Sometimes it is a clear 

 and watery fluid, conglomerating into large drops 

 such as are said to have been observed by Miller 

 of Chelsea exuding from the leaves of the plan- 

 tain, and such as are to be seen in hot and calm 

 weather exuding from the leaves of the poplar, 

 or willow, and trickling down in such abun- 

 dance as to resenible a slight shower. This 

 was observed by Smith under a grove of wiUows 

 in Italy, and is said to occur sometimes even in 

 England. Sometimes it is glutinous, as in the leaf 

 of the lime tree ; sometimes waxy, as in the 

 leaves of the rosemary ; saccharine, as on the 

 orange leaf. On the leaves of the cistus creticus 

 is exuded a resin known by the name of lab- 

 dannm. The exudation from the Lombardy 

 poplar has been rendered famous by Ovid, who 

 fables them as the tears of Phaeton's sisters, who 

 were transformed into this species of poplar. 

 The leaves of fraxinella are also said to be often 

 covered with a sort of resinous substance ; and. 

 after a hot dajr, if the air is calm, the plant is 

 even found to be sun-ounded by a resinous at- 

 mosphere, which may be set on fire by the ap- 

 plication of the flame of a candle. This is 

 said to have been the discoveay of a daughter of 

 the celebrated Linnaeus. Sometimes this ex- 

 udation is a redundancy so gi'eat as to consti- 

 tute a disease of plants; of this nature is tlie 

 honey dew, a sweetish substance exuded by the 

 hop plant, beech tree, &c. 



Roots, as before remarked, also excrete, by 

 their slender extremities, certain fluids, which 

 are injurious or useful to the plants that gTow 

 in their vicinity ; and in this manner, the lik- 

 ings and antipathies of certain plants may be 

 accounted for. Thus, it is well known that the 

 creeping thistle is hurtful to oats, erigeron acris 

 to wheat, scahiosa arrensis to flax, &c. Such 

 are the different phenomena which depend upon 

 the presence of the sa]i, when it has arrived at 

 the upper parts of plants. Let us now follow 

 it in its retrograde course from the leaves to the 

 roots. 



The descending Sap. This has been a subject 

 of much discussion among physiologists, several 

 of them having long denied the existence of a 

 descending sap ; but the perceptible phenomena 

 of \egetation, and tlie most accurate experi- 



ments, have demonstrated that there really is a 

 second sap, which follows a course the reverse 

 of that which we have just examined. If a 

 strong ligature be applied to the trunk of a di- 

 cotyledonous tree, there forms above it a cir- 

 cular swelling, which gradually becomes more 

 prominent. This swelling could not be formed 

 by the sap which ascends from the roots toward 

 the leaves. Were this the case, it ought to pre- 

 sent itself beneath the ligature, and not above it; 

 but this is not what happens. The swelling, 

 therefore, can only depend upon the obstacle 

 which the juices encounter as they descend from 

 the upper parts of the plant to the lower, in 

 their passage through the cortical layers. There 

 is, therefore, a descending sap. 



The descending sap, divested of the greater 

 part of its watery principles, more liigMy ela- 

 borated, and containing more nutritious princi- 

 ples than the ascending sap, contributes essen- 

 tially to the nourishment of the plant. As it 

 circulates in the vegetating part of the stem, 

 the only part susceptible of growth, its uses 

 cannot be equivocal. 



Let us examine more strictly the phenomena 

 which result from the application of a circular, 

 ligature to the trimk of a dicotyledonous tree, 

 and we shall see that not only does a swelhng 

 form above the ligature, biit also that the part 

 of the trunk situated beneath it ceases to grow, 

 no new circular layer being henceforth added to 

 those which previously existed. Hence we see, 

 in the clearest manner, the use of the descending 

 sap. It continually maintains and renews the 

 cambium, and contributes essentially to the 

 growth and development of dicotyledonous 

 trees. 



But this second sap is not of the same nature 

 in aU vegetables. There are some in which it 

 forms a white and milky juice, as in the enphor- 

 biw. In others, as poppies, it is a yellowish or 

 brownish fluid ; and in the firs it is resinous. 

 But it is necessary to remark, that, in the opinion 

 of many physiologists, the proper juices of 

 plants are not the descending sap itself, but fluids 

 which are separated fi-om it by the act of vege- 

 tation. The diversity of nature which these 

 juices present, their occurring in some vegetables 

 only, and their being contained in vessels appro- 

 priated to themselves and existing in small 

 number, appear so many proofs in favour of this 

 opinion. 



We have now given a successive account of 

 the various phenomena which are connected with 

 the nutrition of plants, or contribute to effect 

 it. We have seen the juices which have been 

 absorbed by the roots in the earth conveyed by 

 an inherent power, depending upon the life of 

 the plant, and electric influence, to the highest 

 parts of the ultimate ramifications of the stem. 

 There, v.c ha\'e seen them mingling with the 



