Book I. PROCESS OF VEGETABLE DEVELOPEMENT. 243 



tJiough his doctrine of a circulation should be false, yet the account which lie gives of the 

 progress and agency of the sap and proper juice, short of circulation, may be true. The 

 sum of the account is as follows : When the seed is deposited in the ground under 

 proper conditions, moisture is absorbed and modified by the cotyledons, and conducted 

 directly to the radicle, which is by consequence first developed. But the fluid which has 

 been thus conducted to the radicle, mingling no doubt with the fluid which is now also 

 absorbed from the soil, ascends afterwards to the plumelet through the medium of the 

 tubes of the alburnum. The plumelet now expands and gives the due preparation to the 

 ascending sap, returning it also in its elaborated state to the tubes of the bark, through 

 which it again descends to the extremity of the root, forming in its progress new bark and 

 new alburnum; but mixing also, as he thinks, with the alburnum of the former year, 

 where such alburnum exists, and so completing the circulation, 



1551. Decomposite organs. To the above brief sketch of the agency of the vital 

 principle in the generation or growth of the elementary and composite organs, there now 

 remains to be added that of the progress and mode of the growth of the decomposite or- 

 gans, or organs immediately constituting the plant, as finishing the process of the vegetable 

 developement. This will include the phenomena of the ultimate developement of tlie root, 

 stem, branch, bud, leaf, flower, and fruit. *- 



1552. The root. From the foregoing observations and experiments, it appears that the roots of plants, 

 or at least of woody plants, are augmented in their width by the addition of an annual layer, and in their 

 length by the addition of an annual shoot, bursting from the terminating fibre. But how" is the develope- 

 ment of the shoot effected ? Is it by the intro-susception of additional particles throughout the whole of 

 its extent ; or only by additions deposited at the extremity ? In order to ascertain the fact, with regard 

 to the elongation of the root, Du Hamel instituted the following experiment : Having passed several 

 threads of silver transversely through the root of a plant, and noted the distances, he then immersed the 

 root in water. The upper threads retained always their relative and original situation, and the lowest 

 thread which was placed within a few lines of the end was the only one that was carried down. Hence 

 he concluded that the root is elongated merely by the extremity. Knight, who from a similar exjjeriment 

 obtained the same result, deduced from it also the same conclusion. We may regard it then as certain, 

 that the mode of the elongation of the root is such as is here represented, though in the progress of its 

 developement, it may affect a variety of directions. The original direction of the root is generaly perpen- 

 dicular, in which it descends to a considerable depth if not interrupted by some obstacle. In taking up 

 some young oak-trees that had been planted in a v>oor soil, Du Hainel found that the root had descendeti 

 almost four feet, while the height of the trunk was not more than six inches. If the root meets with an 

 obstacle it then takes a horizontal direction, not by the bending of the original shoot, but by the sending 

 out of lateral shoots. The same effect also follows if the extremity of the root is cut off, but not always 

 so, for it is a common thing in nursery-gardens, to cut off the tap-roots of drills of seedling oaks without 

 removing them, by a sharp spade, and these geiierally push out new tap-roots, though not so strong as the 

 former. When a root ceases of its own accord to elongate, it sends out also lateral fibres which become 

 branches, and are always the more vigorous the nearer they are to the trunk, but the lateral branches of 

 horizontal roots are the less vigorous the nearer they are to the end next the trunk. In the former case, 

 the increased luxuriance is perhaps owing to the easy access of oxygen in the upper divisions ; but in the 

 latter case, the increased luxuriance of the more distant divisions is not so easily accounted for, if it is not 

 to be attributed to the more ample supply of nutriment which the fibres meet with as they recede from 

 the trunk, particularly if you suppose a number of them lying horizontally and diverging like the radii of 

 a circle. But the direction of roots is so liable to be affected by accidental causes, that there is often but 

 little uniformity even in roots of the same species. If plants were to be sown in a soil of the same density 

 throughout, perhaps there might be at least as much uniformity in the figure and direction of their roots, 

 as of their branches ; but this will seldom happen. For if the root is injured by the attacks of insects, or 

 interrupted by stones, or earth of too dense a quality, it then sends out lateral branches, as in the above 

 cases ; sometimes extending also in length by following the direction of the obstacle, and sometimes ceas- 

 ing to elongate, and forming a knot at the extremity. But where the soil has been loosened by digging or 

 otherwise, the root generally extends itself to an unusual length, and where it is both loosened and en- 

 riched, it divides into a multiplicity of fibres. This is also the case with the roots of plants vegetating in 

 pots, near a river, but especially in water. Where roots have some considerable obstacle to overcome they 

 will often acquire a strength proportioned to the difficulty : sometimes they will penetrate through the 

 hardest soil to get at a soil more nutritive, and sometimes they will insinuate their fibres into the crevices 

 even of walls and rocks which they will burst or overturn. This of course requires much time, and does 

 much injury to the plant. Roots consequently thrive best in a soil that is neither too loose nor too dense ; 

 but as the nourishment which the root absorbs is chiefly taken up by the extremity, so the soil is often 

 more exhausted at some distance from the trunk than immediately around it. Du Hamel regards the 

 small fibres of the root which absorb the moisture of the soil as being analogous to the lacteals of the ani- 

 mal system, which absorb the food digested by the stomach. But the root is rather to be regarded as the 

 mouth of the plant, selcctiug what is useful to nourishment and rejecting what is yet in a crude and indi- 

 gestible state ; the larger portions of it serving also to fix the plant in the soil and to convey to the trunk 

 the nourishment absorbed by the smaller fibres, which ascending by the tubes of the alburnum, is thus 

 conveyed to the leaves, the digestive organs of plants. Du Hamel thinks that the roots of plants are fur- 

 nished with pre-organised germs by which they are enabled to send out lateral branches when cut, though 

 the existence of such germs is not j)roved ; and affirms that the extremities of the fibres of the root die 

 annually like the leaves of the trunk and branches, and are again annually renewed ; which last peculiarity 

 Professor Wildenow affirms also to be the fact, but without adducing any evidence by which it apixjars to 

 be satisfactorily substantiated. On the contrary. Knight, who has also made some observations on this 

 subject, says, it does not appear that the terminating fibres of the roots of woody plants die annually, 

 though those of bulbous roots are found to do so. But the fibres of creeping plants, as the common crow- 

 foot and strawberry, certainly die annually, as do those of the vine. 



1553. The stem. The stem, like the root, or at least the stem of woody plants, is also augmented in 

 width by the addition of an annual layer, and in length by the addition of an annual shoot bursting from 

 the terminating bud. Is the developement of the shoot issuing from the stem effected in the same man- 

 ner also ? The developement of tiie .shoot from the stem is not effected in the same manner as that of the 

 root by additions to the extremity only, but by the intro-susception of additional particles throughout its 

 whole extent, at least in its soft and succulent state : the longitudinal extension diminishing in i)ro|)ortion 

 as the shoot requires solidity, and ceasing entirely when the wood is perfectly formed ; though often con- 

 tinuing at the summit after it had ceased at the base. The extension of the shoot is inversely at its indu- 

 ration, rapid while it remains herbaceous, but slow in proportion as it is converted into wood. Hence 

 moisture and shade are the most favorable toils elongation, because they prevent or retard its induration ; 



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