ON THE SEED. 2-15 



Mrs. B. The roots, you must recollect, grow only at 

 their extremities ; and these, being at first of so soft a tex- 

 ture as to be almost liquid, naturally follow the direction 

 of gravity and descend, unless they encounter some ob- 

 stacle, such as a stone or clod of earth, so compact that 

 they cannot penetrate it ; in which case they grow out 

 laterally, in order to avoid what they cannot overcome. 



Mr. Knight performed a very curious experiment, with 

 the view of ascertaining whether it was gravity which 

 made the roots of a plant grow downwards. He sowed 

 seeds in moss disposed in cavities, arranged on the circum- 

 ference of a water-wheel. The cavities were open on both 

 sides, so that the root and the stem were free to germinate 

 at either. The wheel was then made to revolve one hun- 

 dred and sixty times in a minute. The roots invariably 

 struck in the direction diverging from the centre, like the 

 spokes of a wheel : whence Mr. Knight was led to con- 

 clude, that, in this artificial process, the centrifugal force 

 had replaced that of gravity. 



Emily. That was a most ingenious contrivance. But 

 the stem, on the contrary, grows upwards, and throughout 

 its length. 



Mrs. B. Let us suppose that it were free to grow in 

 any direction. Since it shoots from the upper surface of 

 the neck, it cannot grow downwards : it must, therefore, 

 either rise vertically, or shoot out side-ways. In the lat- 

 ter case, it will be gradually brought to a vertical direc- 

 tion by the same cause which makes branches tend to 

 grow upright ; that is to say, the fluids which circulate in 

 the stem having naturally a tendency downwards, some 

 portion, however small, will exude from the upper to the 

 under side of the lateral stem ; so that the lower, being 

 more amply supplied with juices, will vegetate with more 

 vigor, and grow larger. The diminutive upper side Will 

 act like the cord of a bow, and make the stem approxi- 

 mate towards a vertical direction ; and this cause, con- 

 tinuing to act on the stem so long as it is not upright, 

 will ultimately render it erect. 



1338. Why do roots grow downward 1 ? 1339. What experiment 

 on the subject was made by Mr. Knightl 1340. With what result 1 ? 

 1341. In what two directions must the stem necessarily growl 1342. 

 If it commence growing laterally how may it be turned upward? 



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