172 SCIENCE OF GARDENING. Tart II. 



into the leaf-stalk. These were the vessels returning the elaborated sap. The vessels observable on the 

 upper surface Darwin calls arteries, and those on the under surface he calls veins. To this may be added 

 the more recent discoveries of Knight, who in his experiments, instituted with a view to ascertain the 

 course of the sap, detected in the leaf-stalk, not only the vessels which he calls central tubes, through 

 which the colored infusion ascended, together with their appendages, the spiral tubes ; but also another 

 set of vessels surrounding the central tubes, which he distinguishes by the appellation of external tubes, 

 and which appeared to be conveying in one direction or other a fluid that was not colored, but that 

 proved, upon further investigation, to be the descending proper juice. In tracing them upwards they 

 were found to extend to the summit of the leaf, and in tracing them downwards they were found 

 to extend to the base of the leaf-stalk, and to penetrate even into the inner bark. According 

 to Knight, then, there are three sets of vessels in leaves, the central tubes, the spiral tubes, and the 

 external tubes. But by what means is the proper juice conducted from the base of the leaf-stalk to the 

 extremity of the root ? This was the chief object of the enquiry of the earlier phytologists who had not 

 yet begun to trace its progress in the leaf and leaf-stalk ; but who were acquainted with facts indicating 

 at least the descent of a fluid in the trunk. Du Hamel stript sixty trees of their bark in the course of the 

 spring, laying them bare from the upper extremity of the sap and branches to the root; the experiment 

 proved indeed fatal to them, as they all died in the course of three or four years. But many of them 

 had made new productions both of wood and bark from the buds downwards, extending in some cases to 

 the length of a foot ; though very few of them had made any new productions from the root upwards. 

 Hence it is that the proper juice not only descends from the extremity of the leaf to the extremity of the 

 root, but generates also in its descent new and additional parts. The experiments of Knight on this sub- 

 ject are, if possible, more convincing than even those of Du Hamel. From the trunks of a number of 

 young crab-trees he detached a ring of bark of half an inch in breadth. The sap rose in them, and the 

 portion of the trunk above the ring augmented as in other subjects that were not so treated, while the 

 portion below the ring scarcely augmented at all. The upper lips of the wounds made considerable ad- 

 vances downwards, while the lower lips made scarcely any advances upwards ; but if a bud was protruded 

 under the ring, and the shoot arising from it allowed to remain, then the portion of the trunk below that 

 bud began immediately to augment in size, while the portion between the bud and incision remained 

 nearly as before. When two circular incisions were made in the trunk so as to leave a ring of bark be- 

 tween them with a leaf growing from it, the portion above the leaf died, while the portion below the leaf 

 lived ; and when the upper part of a branch was stripped of its leaves the bark withered as far as it was 

 stript. Whence it is evident that the sap which has been elaborated in the leaves and converted into 

 proper juice, descends through the channel of the bark, or rather between the bark and alburnum to 

 the extremity of the root, effecting the developement of new and additional parts. But not only is 

 the bark thus ascertained to be the channel of the descent of the proper juice, after entering the trunk ; 

 the peculiar vessels through which it immediately passes, have been ascertained also. In the language 

 of Knight they are merely a continuation of the external tubes already noticed, which after quitting the 

 base of the foot-stalk he describes as not only penetrating the inner bark, but descending along with it 

 and conducting the proper juice to the very extremity of the root. In the language of Mirbel they are 

 the large or rather simple tubes so abundant in the bark of woody plants, though not altogether confined 

 to it ; and so well adapted by the width of their diameter to afford a passage to the proper juice. 



761. Causes of descent. The proper juice then, or sap elaborated in the leaf, descends 

 by the returning vessels of the leaf-stalk, and by the longitudinal vessels of the inner bark, 

 the large tubes of Mirbel and external tubes of Knight, down to the extremity of the root. 



The descent of the proper juice was regarded by the earlier phytologists as resulting from the 

 agency of gravitation, owing perhaps more to the readiness with which the conjecture suggests itself 

 than to the satisfaction which it gives. But the insufficiency of this cause was clearly pointed out 

 by Du Hamel, who observed in his experiments with ligatures that the tumor was always formed 

 on the side next to the leaves, even when the branch was bent down, whether by nature or art, so 

 as to point to the earth, in which case the power propelling the proper juice is acting not only in 

 opposition to that of gravitation, but with such force as to overcome it. This is an unanswer- 

 able argument ; and yet it seems to have been altogether overlooked, or at least undervalued in its 

 importance by Knight, who endeavours to account for the effect by ascribing it to the joint operation 

 of gravitation, capillary attraction, the waving motion of the tree, and the structure of the conducting 

 vessels; but the greatest of these causes is gravitation. Certain it is that gravitation has considerable 

 influence in preventing the descent of the sap in young shoots of trees which have grown upright, which, 

 when bent down after being fully grown, form larger buds, and often blossom instead of leaf buds. This 

 practice, with a view to the production of blossom-buds is frequently adopted by gardeners (Hort. Trans. 

 i. 237.) in training fruit-trees. These causes are each perhaps of some efficacy; and yet even when 

 taken altogether they are not adequate to the production of the effect. The greatest stress is laid upon 

 gravitation ; but its agency is obviously over-rated, as is evident from the case of the pendent shoots of 

 the weeping willow ; and if gravitation is so very efficacious in facilitating the descent of the proper 

 juice, how comes its influence to be suspended in the case of the ascending sap ? The action of the silver 

 grain will scarcely be sufficient to overcome it ; and if it should be said that the sap ascends through the 

 tubes of the alburnum by means of the agency of the vital principle, why may not the same vital prin- 

 ciple conduct also the proper juice through the returning vessels of the bark. In short if, with Saussure, 

 we admit the existence of a contracting power in the former case sufficient to propel the sap from ring to 

 ring, it will be absolutely necessary to admit it also in the latter. Thus we assign a cause adequate to 

 the production of the effect, and avoid at the same time the transgression of that most fundamental prin- 

 ciple of all sound philosophy which forbids us to multiply causes without necessity. 



Sect. IV. Process of Vegetable Developement. 



762. The production of the different parts and organs of plants is effected by the assimi- 

 lation of the proper juice. The next object of our enquiry, therefore, will be that of 

 tracing out the order of the developement of the several parts, together with the peculiar 

 mode of operation adopted by the vital principle. But this mode of operation is not 

 exactly the same in herbaceous and annual plants, as in woody and perennial plants. In 

 the former, the process of developement comprises as it were but one act of the vital prin- 

 ciple, the parts being all unfolded in immediate succession and without any perceptible 

 interruption till the plant is complete. In the latter, the process is carried on by gradual 

 and definite stages easily cognisable to the senses, commencing with the approach of 

 spring, and terminating with the approach of winter ; during which, the functions of the 

 vital principle seem to be altogether suspended, till it is aroused again into action by the 

 warmth of the succeeding spring. The illustration of the latter, however, involves also 

 that of the former ; because the growth of the first year exemplifies at the same time the 



