Book I. 



PROCESS OF VEGETABLE DEVELOPEMENT. 



175 



modem phytologists. Hedwig is said to have declared himself to be of opmion, that plants 

 have a circulation of fluids similar to that of animals. Corti is said to have discovered a 

 species of circulation in the stem of the chara, but confined, it is believed, witliin the 

 limits of the internodia. Willdenow has also introduced the subject, and defended the 

 doctrine (^Principles of Botany, p. 85.); but only by saying he believes a circulation to 

 exist, and that it is impossible for the leafless tree to resist the cold if there be not a cir- 

 culation of fluids. Knight has given his reasons somewhat in detail ; and though his 

 doctrine of a circulation should be false, yet the account w^hich he 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 follow^s : — When the seed is deposited in the ground under proper condi- 

 tions, moisture is absorbed and modified by the cotyledons, and conducted directly to the 

 radicle, wliich is by consequence first developed. But the fluid vi^hich has been thus con- 

 ducted to the radicle, mingling no doubt with the fluid which is now also absorbed from 

 the soil, ascends afterwards to tlie plumelet through the medium of the tubes of the albur- 

 num. The plumelet now expands and gives the due preparation to the ascending sap, re- 

 turning 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 albur- 

 imm ; but mixing also, as he thinks, with the alburnum of the former year, where such 

 alburnum exists, and so completing the circulation. 



774. Decomposite organs. To the above brief sketch of the agency of the vital prin- 

 ciple in the generation or growth of the elementary and composite organs, there now re- 

 mains to be added that of the progress and mode of the growth of the decomposite organs, 

 or organs inunediately constituting the plant, as finishing the process of the vegetable de- 

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

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



775. 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 experiment 

 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 generally 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 poor soil, Du Hamel found that the root had descended 

 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 generally 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. \Vhere roots have some considerable obstacle to overcome they 

 will often acquire a strength proportioned to the difiSculty : 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 

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

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

 indigestible 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 

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

 though the existence of such germs is not proved ; 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 Profesfsor Willdenow affirms also to be the fact, but without adducing any evidence by which 

 it appears to be satisfactorily substantiated. On the contrary, Knight, who has also made some observ- 

 ations 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 com- 

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



