322 LIFE : OUTLINES OF GENERAL BIOLOGY 



ments from cell to cell, but the larger movements of parts, which 

 are so conspicuous when animals move their appendages, are in- 

 conspicuous and slow in plants. In his highly educative Botany of the 

 Living Plant, Prof. Bower states the case vividly: "One reason for 

 their slowness is no doubt the fact that the protoplast of each cell, 

 though it is the vital agent, is not so free to move as are the proto- 

 plasts of the animal body: for it is encysted by its cell wall. Like the 

 media.»val knight, its movements are checked by its protective 

 armour. The plant has sacrificed mobility for mechanical defence." 

 But we must go further back and note that the initial feature, 

 which even accounts for the protective armour, is the plant's 

 characteristically preponderant anabolism, and this is bound up 

 with the ability to make food out of very simple chemical materials. 

 Growth and Movement. — It is useful to make the contrast that 

 the energy released in the animal body is largely utilised in move- 

 ment and external work, whereas the energy released in the plant 

 body is largely utilised in growth and internal work. But this must 

 be supplemented by the fact that the movements of plants, such as 

 they are, cannot in the majority of cases be separated from growth. 

 In other words, plant movements are commonest in parts that are 

 still young and growing, whereas the most striking movements of 

 animals, such as the flight of birds and insects, are exhibited by 

 finished structures. We shall begin our brief survey, then, by 

 referring to the growth-movements of shoots arid roots and the like. 

 Nutation. — Along a plant's axis (root and stem) there is in- 

 equality of growth at different regions, and the same is seen around 

 the axis in different segments. But the area of most rapid growth 

 shifts, either spontaneously or under known stimulus, from one 

 radial segment to another, and this is expressed in that bending and 

 bowing to different points of the compass which is called nutation. 

 It is exhibited by the free-growing tips of stems and roots, which 

 describe, at varied rates, irregular circles or ellipses or spirals. 



Similarly a young leaf in the bud grows more rapidly on its outer 

 (future under) surface, and this presses it inwards against the young 

 shoot. Later on, the more rapid growth is on the inner (future upper) 

 surface, and this presses the leaf outwards. In short, the movements 

 and foldings of young leaves within the bud, and the movements 

 and unfoldings as the bud opens, are familiar diverse expressions 

 of differential growth. 



Tkopisms. — Some growth-movements have their direction 

 determined by the direction of the incident stimulus, and to these 

 the term tropism is applied. They are obligatory movements whose 

 direction is defined by the stimulus. Thus stems grow in a direction 

 opposite to that of the gravity pull, while roots grow towards the 

 centre of the earth — a familiar fact expressed technically by attribut- 

 ing negative geotropism to the stem and positive geotropism to the 



m. 



