Fronds of Asplenium bulbiferum. 



145 



All these movements are slight ; the first almost imperceptible to the 

 unaided eye, the second merely occasional, and the third only occurs when 

 growth is nearly over. They will, therefore, not concern us further, and we 

 are left with the three most important movements, due to the following 

 causes : — 



(4) Epmasty, present throughout the first two periods, causes more or less 

 striking curvatures towards the end of the adolescent phase. The tip of the 

 frond, which may or may not be entirely uncurled, approaches the ab-axial 

 side so that the upper part forms a loop, sometimes almost a complete circle, 

 a position from which it eventually recovers by slight hyponasty. 



(5) Light has a directive effect upon the frond which varies with the position 

 of the latter, and does not seem to come into play till it is several centimetres 

 in height, i.e., in the middle of the first phase. Curvature is shown most 

 strongly if the frond is placed abaxially to the light, when the apical part 

 generally moves through 90° in the direction of the incident rays (see 

 fig. 2, A). 



Fig. 2. Fig. 3. 



Fig. 2. — Explanation in text. (Imagine plane of paper vertical.) 



Fig. 3. — Explanation in text. (Direction of light through plane of paper, which is 

 imagined vertical.) 



The effect is much less pronounced, and seems to come into play later, if 

 the frond is placed so that the light strikes it laterally {i.e., at right angles to 

 the plane of the paper, fig. 2), when a slight positive movement takes place in 

 the first period, and diaheliotropism, caused by a torsion of the rachis, is 

 shown by the leaflets in the second. If, however, the frond is placed so that 

 the incident rays strike the adaxial surface (fig. 2, B) little or no curvature 

 takes place, but the frond grows approximately vertically well into the 

 adolescent stage, since the effects of light and epinasty balance one another. 

 The backward curve produced by the latter in the second period is 

 considerably lessened. 



(6) Geotropism. — Irritability to gravity exists from the inception of the 

 frond till near the end of the second phase. It is best studied in fronds 

 placed horizontally and adaxially to the incident light, when the organ will 



