GROWTH, IRRITABILITY AND MOVEMENT 



167 



associated with their divergent methods of securing nutrition. The 

 animal goes in search of its food : while the raw materials which the 

 plant requires for its nutrition reach it by processes of diffusion. Such 

 movements as are exhibited by the individual organs of the plant 

 involve a quite different mechanism from that in animals. With the 

 exception of the hygroscopic movements, they are all dependent on 

 variations of the osmotic turgor of living cells or of the resistance 

 of cell-walls ; it is structurally impossible that the mechanical 

 effects of the movements should exceed, as they mostly fall 

 short of, the limit of the pressure of the protoplast upon the con- 



A -3 



Fig. 103. 



Fern sporangia : A, with the cells of the annulus darkly shaded, and curved 

 strongly backwards by drying of its cells. B, the annulus after its sudden recovery, 

 while the previous position is shown in dotted lines. C shows in detail cells of the 

 annulus in A, and D shows similar cells in the state seen in B. See Chapter XXXI. 



taining walls. Such movements of the plant-body are brought about 

 in an essentially different way from those positive contractions of 

 muscle-fibres, which are the source of movement in the animal body. 

 In this, as in so many other features, the two kingdoms show evidence 

 of their initial divergence. However parallel their behaviour may 

 appear to be, when fully analysed it becomes apparent that it is 

 analogy, rather than any closer correspondence, that holds between 

 them. For we do not find either that contractile muscular fibres 

 exist, or any specialised nerve-system in the body of the plant. 

 Such movements as plants show, and even the conveyance of stimuli, 

 are brought about without them. 



Two other types of movement are encountered within the plant king- 

 dom. Locomotion is displayed by some Algae (Chapters XXI.-XXII.) 

 and by ciliated reproductive cells, such as the sperm of the Fern 

 (Chapter XXXI.). Movement of cell contents also is common in plants. 

 It usually takes the form of protoplasmic streaming (Chapter III.). 



