240 SCIENCE OF AGRICULTURE. Part II. 



by means of the several processes that have just been described, it assumes the appel- 

 lation of the cambium, or proper juice of the plant. In this ultimate state of elaboration 

 it is found chiefly in the bark, or rather between the bark and wood, and may very often 

 be distinguished by a peculiar colour, being sometimes white, as in the several species of 

 spurge, and sometimes yellow, as in celandine. It is said to be the principal seat of the 

 medical virtues of plants ; and was regarded by Malpighi as being to the plant what the 

 blood is to the animal body, the immediate principle of nourishment and grand support 

 of life ; which opinions he endeavours to establish by the following analogies : if the blood 

 escapes from the vessels of the animal body, it forms neither flesh nor bone, but tumours; 

 if the proper juices of the plant are extravasated, they form neither bark nor wood, but a 

 lump of gum, resin, or inspissated juice. The disruption of the blood-vessels, and conse- 

 quent loss of blood, injure and often prove fatal to the animal; the extravasation of 

 the proper juice injures and often proves fatal to vegetables, unless the evil is prevented 

 by the skill and management of the gardener. Whatever may be the value of these re- 

 marks as tending to establish the analogy in question, it cannot be doubted that the cam- 

 bium, or proper juice, constitutes at least the grand principle of vegetal^le organisation ; 

 generating and developing in succession the several organs of the plant, or furnishing the 

 vital principle with the immediate materials of assimilation. 



1562. The proper juice is conveyed to the several parts of the plant by an appropriate set of vessels. One 

 of the earliest and most satisfactory experiments on this subject, at least as far as regards the return of 

 the proper juice through the leaf and leaf-stalk, is that of Dr. Darwin, which was conducted as follows : 

 a stalk of the uph6rb?a helioscopia, furnished with its leaves and seed-vessels, was placed in a decoction 

 of madder-root, so as that the lower portion of the stem and two of the inferior leaves were immersed in 

 it. After remaining so for several days the colour of the decoction was distinctly discerned passing along 

 the midrib of each leaf On the upper side of the leaf many of the ramifications, going from the midrib 

 towards the circumference, were observed to be tinged with red ; but on the under side there was ob- 

 served a sj'stem of branching vessels, originating in the extremities of the leaf, and carrying not a red but 

 a pale milky fluid, which, after uniting in two sets, one on each side the midrib, descended along with it 

 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 coloured 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 which was not coloured, but which 

 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 trunk 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 the 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 

 advances downwards, while the lower lips made scarcely any advances upwards ; but if a bud were 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. 



1563. Causes of descent. The proper juice then, or sap elaborated in the leaf, de- 

 scends 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 extre- 

 mity of the root. 



1564. 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 insufliciency of this cause was clearly pointed out by Du Hamel, 

 who observed in his experiments with ligatures that the tumour 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 unanswerable 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 j but the greatest of these causes is gra- 



