CHAP. TIL] the Movements of Plants. 539 



acuteness in attempts at explaining them ; to these attempts we 

 shall return in a future page. 



A still more universal phenomenon than the vertical growth 

 of stems and roots is the growth of plants generally, and it 

 required as much or even more of the spirit of enquiry to pro- 

 pose the question, whether this growth can be explained by 

 mechanical laws, and what that explanation is. Mariottc 

 touched on this question in 1679, Dut only incidentally, and 

 supposed that the stretching of the pith, which meant at that 

 time the whole of the parenchymatous tissue, was the cause of 

 the growth of the parts of plants. This idea might have had 

 its origin in the Aristotelian notion that the pith is the seat of 

 the vegetable soul, but Marietta endeavoured to give physical 

 reasons for it. Hales in his 'Statical Essays ' of 1727 went 

 much more minutely into the question of the growth of plants. 

 Following the train of thought in his doctrine of the nutrition 

 of plants, he introduces his observations on their growth with 

 the remark, that plants consist of sulphur, volatile salts, earth, 

 water, and air, the first four of which attract one another, and 

 therefore form the solid, inert part of the substance of plants ; 

 the air behaves in a similar manner as long as it is kept by the 

 other substances in a solid condition ; but as soon as it is set 

 at liberty it is capable of expansion. On this power of expan- 

 sion in the air, by which the juices of plants are quickened and 

 strengthened, he builds his mechanical theory of growth, accord- 

 ing to which the plastic parts of the plant assume a state of 

 tension, and as the air enters into combination with other sub- 

 stances and so becomes fixed, warmth and movement are 

 excited, and these make the particles of sap assume by degrees 

 a form and shape. These principles supplied his starting-point. 

 To get a clearer idea of the way in which the growth of the parts 

 of plants proceeds, he made equi-distant punctures in young 

 stalks and leaves, and found that the intervals between them 

 increased by growth more in the younger intervening parts 

 than in the older. In the course of these observations he is 



