第86章

After an elucidation and defence of this position, Mr. Hinton proceedsto interpret, in conformity with it, sundry phenomena of development. Shakingof plants, he says:-- "The formation of the root furnishes a beautiful illustration ofthe law of least resistance, for it grows by insinuating itself, cell bycell, through the interstices of the soil; it is by such minute additionsthat it increases, winding and twisting whithersoever the obstacles it meetsin its path determine, and growing there most, where the nutritive materialsare added to it most abundantly. As we look on the roots of a mighty tree,it appears to us as if they had forced themselves with giant violence intothe solid earth. But it is not so; they were led on gently cell added tocell, softly as the dews descended, and the loosened earth made way. Onceformed, indeed, they expand with an enormous power, but the spongy conditionof the growing radicles utterly forbids the supposition that they are forcedinto the earth. Is it not probable, indeed, that the enlargement of the rootsalready formed may crack the surrounding soil, and help to make the intersticesinto which the new rootlets grow? * * *

"Throughout almost the whole of organic nature the spiral form ismore or less distinctly marked. Now, motion under resistance takes a spiraldirection, be seen by the motion of a body rising or falling through water.

A bubble rising rapidly in water describes a spiral closely resembling acorkscrew, and a body of moderate specific gravity dropped into water maybe seen to fall in a curved direction, the spiral tendency of which may bedistinctly observed. * * * In this prevailing spiral form of organic bodies,therefore, it appears to me, that there is presented a strong prima faciecase for the view I have maintained. * * * The spiral form of the branchesof many trees is very apparent, and the universally spiral arrangement ofthe leaves around the stem of plants needs only to be referred to. * * *The heart commences as a spiral turn, and in its perfect form a manifestspiral may be traced through the left ventricle, right ventricle, right auricle,left auricle, and appendix. And what is the spiral turn in which the heartcommences but a necessary result of the lengthening, under a limit, of thecellular mass of which it then consists? * * *"Every one must have noticed the peculiar curling up of the youngleaves of the common fern. The appearance is as if the leaf were rolled up,but in truth this form is merely a phenomenon of growth. The curvature resultsfrom the increase of the leaf, it is only another form of the wrinkling up,or turning at right angles by extension under limit.

"The rolling up or imbrication of the petals in many flower-budsis a similar thing; at an early period the small petals may be seen lyingside by side; afterwards growing within the capsule, they become folded roundone another. * * *

"If a flower-bud be opened at a sufficiently early period, the stamenswill be found as if moulded in the cavity between the pistil and the corolla,which cavity the anthers exactly fill; the stalks lengthen at an after period.

I have noticed also in a few instances, that in those flowers in which thepetals are imbricated, or twisted together, the pistil is tapering as growingup between the petals; in some flowers which have the petals so arrangedin the bud as to form a dome (as the hawthorn; e.g.), the pistil is flattenedat the apex, and in the bud occupies a space precisely limited by the stamensbelow, and the enclosing petals above and at the sides. I have not, however,satisfied myself that this holds good in all cases."Without endorsing all Mr. Hinton's illustrations, his conclusion may beaccepted as a large instalment of the truth. But in the case of organic growth,as in all other cases, the line of movement is in strictness the resultantof tractive and resistant forces; and the tractive forces here form so considerablean element that the formula is not complete without them. The shapes of plantsare manifestly modified by gravitation. The direction of each branch is notwhat it would have been in the absence of the pull exercised by the Earth;and every flower and leaf is somewhat altered in the course of developmentby the weight of its parts. Though in animals such effects are less conspicuous,yet the instances in which flexible organs have their directions in greatmeasure determined by gravity, justify the assertion that throughout thewhole organism the forms of parts must be affected by this force.

The organic movements which constitute growth, are not, however, the onlyorganic movements to be interpreted. There are also those which constitutefunction; and throughout these the same general principles are discernible.

That the vessels and ducts along which blood, lymph, bile, and all the secretions,find their ways, are channels of least resistance, is an illustration almosttoo conspicuous to be named. Less conspicuous, however, is the truth thatthe currents set ting along these vessels are affected by the tractive forceof the Earth; witness varicose veins; witness the relief to an inflamed partobtained by raising it; witness the congestion of head and face producedby stooping. And in the facts that dropsy in the legs gets greater by dayand decreases at night, while, conversely that oedematous fullness underthe eyes common in debility, grows worse during the hours of reclining anddecreases after getting up, we see how the transudation of liquid throughthe walls of the capillaries, varies according as change of position changesthe effect of gravity in different parts of the body .