[VEGETABLE PHYSIOLOGY. 



external cause, and not by any power inherent 

 in vegetable life; and I see little reason to 

 doubt that gravitation is the principal, if not 

 the only, agent employed in this case by nature." 

 Mr. Knight has endeavoured to point out the 

 means by which he conceives the same agent 

 may produce effects so diametrically opposite 

 to each other. 



It has, however, been objected by Duhamel 

 (and the greatest deference is always due to 

 his opinions) that gravitation could have little 

 influence on the direction of the germen, were 

 it, in the first instance, protruded, or were it 

 subsequently inverted, and made to point per- 

 pendicularly downwards. To enable myself, 

 says Mr. Knight, to answer this objection, I 

 made many experiments on trees of the horse- 

 chestnut and of the bean, in the box I have 

 already described ; and as the seeds there were 

 suspended out of the earth, I could regularly 

 watch the progress of every effort made by the 

 radicle and germen to change their positions. 

 The extremity of the radicle of the bean, when 

 made to point perpendicularly upwards, gene- 

 rally formed a considerable curvature within 

 3 or 4 hours when the weather was warm. 

 The germen was more sluggish ; but it rarely 

 or never failed to change its direction in the 

 course of 24 hours ; and all my efforts to make 

 it grow downwards by slightly changing its 

 direction were invariably abortive. 



As trees possess the power of turning the 

 upper surfaces of their leaves and the points 

 of their shoots to the light, and their tendrils in 

 any direction to attach themselves to conti- 

 guous objects, it may be suspected that their 

 lateral roots are by some means directed to any 

 soil in their vicinity which is best calculated 

 to nourish the plant to which they belong ; and 

 it is well known that much the greater part of 

 the roots of an aquatic plant which has grown 

 in a dry soil, on the margin of a lake or river, 

 have been found to point to the water, whilst 

 those of another species of tree which thrives 

 best in a dry soil have been ascertained to take 

 an opposite direction : but the result of some 

 experiments I have made is not favourable to 

 this hypothesis ; and I am inclined to believe 

 that the roots disperse themselves in every 

 direction, and only become more numerous 

 where they find most employment, and a soil 

 best adapted to the species of plant. 

 ' A tree growing upon a wall at some distance 

 from the ground, and consequently ill supplied 

 with food and water, has also been observed to 

 adapt its habits to its situation, and to make 

 very singular and well-directed efforts to reach 

 the soil beneath by means of its roots. Dur- 

 ing the period in which it is making such 

 efforts, little addition is made to its branches, 

 and almost the whole powers of the plant ap- 

 pear to be directed to the growth of one or 

 more of its principal roots. To these much in 

 consequence is annually added, and they pro- 

 ceed perpendicularly towards the earth, unless 

 made to deviate by some opposing body; and 

 as soon as the roots have attached themselves 

 to the soil, the branches grow with vigour and 

 rapidity, and the plant assumes the ordinary 

 habits of its species. 



In some other experiments of Knight to illus- 

 1078 



VEGETABLE PHYSIOLOGY. 



| trate these highly interesting habits of plants, 

 pieces of alum, and of the sulphate of iron 

 | (green vitriol), blue vitriol (sulphate of cop- 

 j per), were placed at small distances perpendi- 

 ! cularly beneath the radicles of germinating 

 j seeds of different species, to afford an oppor- 

 tunity of observing whether any efforts could 

 be made by them to avoid poisons ; but they 

 did not appear to be at all influenced except by 

 actual contact of the injurious substances. 

 The growth of their fibrous lateral roots was, 

 however, obviously accelerated when their 

 points approached any considerable quantity 

 of decomposing vegetable or animal matter; 

 and when the growth of the roots was retarded 

 by want of moisture, the contiguity of water 

 in the adjoining mould, though not apparently 

 in actual contact with them, operated benefi- 

 cially: but I had reason to suspect that the 

 growth of roots was, under these circum- 

 stances, promoted by actual contact with the 

 detached and fugitive particles of the decom- 

 posing body and the evaporating water. 



The way in which plants establish them- 

 selves in opposition to the various obstacles 

 they have to encounter, as, for instance, in, 

 withstanding violent winds, is very remarkable. 

 The growth and forms assumed by the roots 

 of trees of every species are, to a great extent, 

 dependent upon the quantity of motion which 

 their stems and branches receive from winds ; 

 for the effects of motion upon the growth of 

 the root and of the trunk and branches are 

 perfectly similar. Whatever part of a root is 

 moved and bent by winds or other causes, an 

 increased deposition of alburnous matter upon 

 that part soon takes place, and consequently 

 the roots which immediately adjoin the trunk 

 of an insulated tree in an exposed situation be- 

 come strong and rigid, whilst they diminish 

 rapidly in bulk as they recede from the Jrunk, 

 and descend into the ground ; by this sudden 

 diminution of the bulk of the roots the passage 

 of the descending sap through their bark is ob- 

 structed, and it, in consequence, generates, and 

 passes into many lateral roots, and these, if the 

 tree be still much agitated by winds, assume a 

 similar form, and consequently divide into 

 many others. A kind of net-work, composed 

 of thick and strong roots, is thus formed, and 

 the tree is secured from the danger to which 

 its situation would otherwise expose it. In a 

 sheltered valley, on the contrary, where a tree 

 is surrounded and protected by others, and is 

 rarely agitated by winds, the roots grow long 

 and slender, like the stem and branches, and 

 comparatively much less of the circulating 

 fluid is expended in the deposition of alburnum 

 beneath the ground ; and hence it not unfre- 

 quently happens that, a tree in the most shel- 

 tered part of a valley is uprooted, whilst the 

 exposed and insulated tree upon the adjoining 

 mountain remains uninjured by the fury of the 

 storm. 



All such investigations as these are fraught 

 with instruction to the cultivator of the earth. 

 They not only illustrate the every-day opera- 

 tions of the farmer, but they guard him against 

 the adoption of specious novelties and unsci- 

 entific efforts to increase the fertility of the 

 ; soil. Such researches, too, will hardly fail to 



