CHAP. XII. CONCLUDING EEMARKS. 559 



We have now to consider circumnutation in a 

 modified form, as the source of several great classes of 

 movement. The modification may be determined by 

 innate causes, or by external agencies. Under the first 

 head we see leaves which, when first unfolded, stand 

 in a vertical position, and gradually bend downwards 

 as they grow older. We see flower-peduncles bending 

 down after the flower has withered, and others rising 

 up ; or again, stems with their tips at first bowed 

 downwards, so as to be hooked, afterwards straighten- 

 ing themselves ; and many other such cases. These 

 changes of position, which are due to epinasty or 

 hyponasty, occur at certain periods of the life of the 

 plant, and are independent of any external agency. 

 They are effected not by a continuous upward or 

 downward movement, but by a succession of small 

 ellipses, or by zigzag lines, that is, by a circum- 

 nutating movement which is preponderant in some 

 one direction. 



Again, climbing plants whilst young circumnutate 

 in the ordinary manner, but as soon as the stem 

 has grown to a certain height, which is different for 

 different species, it elongates rapidly, and now the 

 amplitude of the circumnutating movement is im- 

 mensely increased, evidently to favour the stem catch- 

 ing hold of a support. The stem also circumnutates 

 rather more equally to all sides than in the case of 

 non-climbing plants. This is conspicuously the case 

 with those tendrils which consist of modified leaves, 

 as these sweep wide circles ; whilst ordinary leaves 

 usually circumnutate nearly in the same vertical plane. 

 Flower-peduncles when converted into tendrils have 

 their circumnutating movement in like manner greatly 

 increased. 



We now come to our second group of circumnu- 



