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soil, air and water, but occur in irregular masses. In order to acquire 

 these separate portions of food it was and is highly necessary that the 

 animal should be able to move from place to place, and should be able 

 to sustain this movement sufficiently to acquire the requisite number of 

 food-masses ; hence arose the necessity for the power of locomotion, 

 which also serves the animal as a means of protection from danger from 

 other animals, inclemencies of the climate, etc. 



The emergence of the plant from an aquatic to a terrestrial habitat in an 

 early stage of its development was accompanied by several radical changes 

 in its physiological organization, and from a motile, or floating body it 

 acquired the habit of fixing itself firmly in the soil or other substatum. 

 The power of locomotion was not only useless but impossible in its new 

 location, and with its newly acquired rigid body, and hence it was lost. 



The loss of the power of movement from place to place and the acquisi- 

 tion of the habit of fixation was due to the character of the food supply. 



The food of plants consists of mineral salts derived chiefly from the 

 soil and carbon dioxide from the air. 



The mineral elements and water are quite widely distributed through- 

 out the soil, and furthermore in sufficient quantity to enable a plant to 

 meet its needs without moving from the place in which it began its 

 existence. It is but necessary that the roots should pierce the soil and 

 place the absorbing surfaces in contact with the solutions serving as 

 food. The mineral food of the plant thus lies in the soil beneath it or 

 near it laterally, and the physical conditions prevalent make something 

 more necessary than a simple random penetration of the soil. In other 

 words it is necessary that the root tips should be guided as they bore 

 through the substratum. In order to guide or direct the growth of a 

 tip of a root, it is necessary that this tip should have the power of 

 movement or of changing the position of its axis. Thus it is of the 

 greatest importance that the primary root of a seedling should penetrate 

 the soil in a vertical direction and reach the moist particles at greater 

 or less distance beneath the surface. To accomplish this the primary 

 root should always grow downward no matter in what position it may be 

 placed at the time of the germination of the seed. In order to do this 

 the root has acquired the power of irritability to gravity which we term 

 geotropism, A geotropic root tends to place its axis parallel to the force 

 of gravity, that is with its point directed toward the centre of the earth. 

 The movement by which the plant accomplishes this may be very easily 

 demonstrated if a seedling with a root two or three inches in length is 

 placed in such position that the root is in a horizontal position in a 

 damp chamber. If examined an hour later it will be found that curva- 

 ture has taken place in a portion near the apex and that the tip now 

 points directly downward. 



It would not suffice however for all of the root to be driven directly 

 downward through the soil, since the soil containing the most ad- 

 vantageous proportions of food, lies comparatively near the surface. 

 The lateral roots which issue from the primary roots are therefore en- 

 dowed with a power of movement which tends to place their axis in a 

 horizontal position. By a combination of the two movements it may be 

 seen that the plant is enabled to drive its roots downward to the proper 

 depth and laterally through the layer of the best soil. The difficulties 

 to be overcome and the conditions to be met in this penetration of the 



