PRESIDENT S ADDRESS. XIX 



constantly circumnutating. In climbing plants of vigorous growth 

 the circumnutating movement is immensely increased, evidently 

 to enable the stem to catch hold of a support. Secondary and 

 tertiary radicles are gifted with somewhat different powers, but 

 in all of them the point of extreme sensitiveness resides in the 

 tip. I took up a primrose root the other day which had been 

 planted about a fortnight, and found it covered in the mean- 

 time with young rootlets. Imagine this multitude of delicate 

 white threads, each of them carefully searching amidst the 

 coarse earth for the path of least resistance, and it will give us 

 an idea of the wonderful mechanism of even these out-of-sight 

 parts of plants. In the sleep of plants leaves often arrange them- 

 selves in a variety of positions, m many instances evidently in 

 order to avoid the effects of radiation, which would act more 

 powerfull}^ and produce greater cold were the leaves in a horizon- 

 tal position. This is also regarded as a modified circumnutation. 

 There is another class of movement in leaves, that which 

 causes them to change their position and present their edges 

 to the light during the brightest period of the day, which is some- 

 times called the dim-nal sleep of plants. In a species of Silphium 

 during the devolopmeut of the leaves the petiole is twisted so 

 that the blades face east and west and their edges north and 

 south. The flower heads are almost always turned eastwards. 

 Sir J. Hooker states that in travelling by rail any alteration in 

 the direction of the road becomes apparent by the altered 

 appearance of the leaves of the " compass plant." 



These are some of the movements which have been observed 

 in plants, but the most striking resemblance to the movements 

 of animals occurs when an influence is transmitted from one 

 part of a plant on its being excited to another more or less 

 distant, which moves in response, as is the case of the sensitive 

 Mimosa, the Dionsa, and other plants. Plants do not possess 

 a nen^ous system, and movements must consequently be trans- 

 mitted by an alteration in the condition and consequently in 

 the form of the cells, but why a touch should cause a change, 

 and why its eflect should travel so instantaneously is not known. 



