THR ROOT. 109 



whoat wouKI not grow ; hence in transplanting trees, the above 

 principles would guide us to seek plants of' a ditKerent family 

 from llioic that previoiKsly occupied the ground. It might 

 perhaps he thought that trees would vitiate the soil in which 

 thev iTrow so as lo become their own destroyers, but this is pre- 

 vented by the arrangement, that the roots absorb only by their 

 extremities, and these are constantly lengthening and of course 

 chan'MUOf ihoir uosition and cominjj in contact with new earth. 

 The nature of ihe excrL-ted subst.mces areol course as various 

 as the lamdies of plants. '* Leguminous plants produce 

 a substance analogous to gum with a little carbonate of lime; 

 grains a minute q lantity of matter containing certain alka- 

 line and earlliy uiuriates and carbonates, but very little gum ;" 

 others tlie bitter principle and substance resem!)ling opium, 

 containing tannin and other salts. Our space forbids speci- 

 fvin<»' tie numerous excretions of the ditiurent orders, which 

 have been determined. Tiie pra("tical application of liiese 

 principles is important to the Agriculturalist and Horticultural. 

 ist ; it teaches the first that the greater the variety of crops he 

 can proJ ice on his plantation, the belter they will be, 

 and the less likely to he injured by attacks peculiar to the 

 crop. It teaches the latter the beaefir, if not the necessity, 

 of chanjring the compartments of his garden in succession 

 for ditf rent productions, and the necessity of changing the 

 earth in pots and boxes occasionally, if he expects plants to 

 continue to thrive, 



134. We have mentioned before that the root always descends 

 in its course of development, unless obstructed by physical 

 impediments. Numerous experiments have been made which 

 strikingly exhibit this vital impulse. To DulrocheF, more 

 than to any other man, is the world indebted for placing this 

 subject in its present interesting position. In several memoirs 

 he has discussed the subject in its various connections, and 

 from his numerous experiments we arrive at the conclusion that. 

 Quantity of matter seems to exercise the most 'Powerful influ- 

 ence in controllinjj the direction of the root. When the root 

 has its situation naturallv in the earth it dc'^cends perpendic- 

 ularly to its surface. Previous to his investigations various 

 hypotheses were abroad to account for the uniform direction 

 of the root and stem; but all were unsatisfactory. If seeds 

 are permitted to sprout in a box, and after the plumula and 

 radicle are developed in their common directions the box bo 

 inverted, the root will change its direction downwards, and 

 the plumula upwards, and if they are permitted to sprout in a 

 tube in which they cannot turn after inversion, they will as- 



10 



