502 MODIFIED CIRCUMNUTATION. CHAP. X 



10.40 P.M., during which time it zigzagged and described an 

 ellipse of considerable size. The hypocotyl of Brassica okracea 

 (see former Fig. 173) moved in a straight line to the light until 

 5.15 P.M., and then from the light, making in its backward 

 course a great rectangular bend, and then returned for a short 

 distance towards the former source of the light ; no observa- 

 tions were made after 7.10 P.M., but during the night it re- 

 covered its vertical position. A hypocotyl of Cassia tora moved 

 in the evening in a somewhat zigzag line towards the failing 

 light until 6 P.M., and was now bowed 20 from the perpendi- 

 cular ; it then returned on its course, making before 10.30 P.M. 

 four great, nearly rectangular bends and almost completing an 

 ellipse. Several other analogous cases were casually observed, 

 and in all of them the apogeotropic movement could be seen to 

 consist of modified circumnutation. 



Apogeotropic Movements effected by the aid of joints or pulvini. 

 Movements of this kind are well known to occur in the 

 Graminese, and are effected by means of the thickened bases 

 of their sheathing leaves; the stem within being in this part 

 thinner than elsewhere.* According to the analogy of all other 

 pulvini, such joints ought to continue circumnutating for a 

 long period, after the adjoining parts have ceased to grow. We 

 therefore wished to ascertain whether this was the case with 

 the Graminese ; for if so, the upward curvature of their stemsj 

 when extended horizontally or laid prostrate, would be explained 

 in accordance with our view namely, that apogeotropism 

 results from modified circumnutation. After these joints have 

 curved upwards, they are fixed in their new position by increased 

 growth along their lower sides. 



Lolium perenne. A young stem, 7 inches in height, consist- 

 ing of 3 internodes, with the flower-head not yet protruded, 

 was selected for observation. A long and very thin glass fila- 

 ment was cemented horizontally to the stem 'close above the 

 second joint, 3 inches above the ground. This joint was subse- 

 quently proved to be in an active condition, as its lower side 

 swelled much through the action of apogeotropism (in the 

 manner described by De Vries) after the haulm had been 

 fastened down for 24 h. in a horizontal position. The pot was 



* This structure has been re- die Aufrichtung des gelagerten 

 cently described by De Vries in Getreides,' in * Landwirthschaft- 

 an interesting article, * Ueber Hche Jahrbiicher,' 1880, p. 473. 



