SECTION 16.] MOVEMENTS. 151 



464. Root-movements. In its growth a root turns or bends away 

 from the light and toward the centre of the earth, so that in lengthening 

 it buries itself in the soil where it is to live and act. Every one must 

 have observed this in the germination of seeds. Careful observations have 

 shown that the tip of a growing root also makes little sweeps or short 

 movements from side to side. By this means it more readily insinuates 

 itself into yielding portions of the soil. The root-tips will also turn 

 toward moisture, and so secure the most favorable positions in the soil. 



465. Stem- movements. The root end of the caulicle or first joint of 

 stem (that below the cotyledons) acts like the root, in turning downward 

 in germination (making a complete bend to do so if it happens to point 

 upward as the seed lies in the ground), while the other end turns or 

 points skyward. These opposite positions are taken in complete dark- 

 ness as readily as in the light, in dryness as much as in moisture : there 

 fore, so far as these movements are physical, the two portions of the same 

 interuode appear to be oppositely aifected by gravitation or other in- 

 fluences. 



466. Rising into the air, the stem and green shoots generally, while 

 young and pliable, bend or direct themselves toward the light, or toward 

 the stronger light when unequally illuminated ; while roots turn toward the 

 darkness. 



467. Many growing stems have also a movement of Nutation, that is, 

 of nodding successively in different directions. This is brought about by 

 a temporary increase of turgidity of the cells along one side, thus bowing 

 the stem over to the opposite side ; and this line of turgescence travels 

 rouud the shoot continually, from right to left or from left to right accord- 

 ing to the species : thus the shoot bends to all points of the compass in 

 succession. Commonly this nutation is slight or hardly observable. It is 

 most marked in 



46S. Twining Stems (Fig. 90). The growing upper end of such 

 stems, as is familiar in the Hop, Pole Beans, and Moruing-Glory, turns 

 over in an inclined or horizontal direcl ion, thus stretching out to reach a 

 neighboring support, and by the continual change in the direction of the 

 nodding, sweeps the whole circle, the sweeps being the longer as the stem 

 lengthens. When it strikes against a support, such as a stem or branch of 

 a neighboring plant, the motion is arrested at the contact, but continues 

 at the growing apex beyond, and this apex is thus made to wind spirally 

 around the supporting body. 



469. Leaf-movements are all but universal. The presentation by 

 most leaves of their upper surface to the light, from whatever direction 

 that may come, is an instance ; for when turned upside down they twist or 

 bend round on the stalk to recover this normal position. Leaves, and the 

 leaflets of compound leaves, change this position at nightfall, or when the 

 light is withdrawn ; they then take what is called their sleeping posture, 

 resuming the diurnal position when daylight returns. This is very striking 



