ON THE POWER OF MOVEMENT IN PLANTS. 15? 



Our last instance of Periodic or regular motion shall be taken 

 from the phenomenon known as Heliotropism^ or turning either 

 towards or away from the light. 



Positive Heliotropism^ or turning towards the sun, is common 

 enough, and is seen in the internodes of growing stems, in petioles, 

 and peduncles or flower stalks. Here, the part turned to the 

 light — i.e.^ the concave side — is retarded in growth ; the convex side — 

 or part turned away from the light — grows more rapidly, a remark- 

 able case of inequality of growth. Among flowers, the Compositce. 

 furnish us with many examples, one being specially prominent — 

 the Sun-flower — whose peduncle twists in a circle during the day, 

 bringing its flower constantly toivards the sun. In the old Roman 

 mythology, Clytie is made to follow Sol wherever he goes, in the 

 form of a flower called Helioti- opium. We do not know what this 

 was. It was not our Heliotrope, because Ovid says it resembles a 

 violet in its form ; it was not our Sunflower, since this was a native 

 of America only, and, therefore, unknown to the Roman writer. 

 Ripening Corn inclines to the South, not to the North. 



Negative Heliotropism — i.e.^ turning away from the light — is a 

 rarer phenomenon, reversing the above process, the part exposed 

 to the sun growing more rapidly, and therefore being convex^ the 

 coficave side being turned away from the sun. Cases are seen in 

 the older branches of the Ivy-plant, and in Vine-tendrils, by means 

 of which process these are enabled to cling to their support ; 

 also, in the tendrils of Big7ionia^ the flower stalks of Cyclame7i^ 

 and the plasmodia, or moving masses of ^thalium. In Cyclameri^ 

 it enables the plant to scoop a hole with its peduncle in the earth, 

 or sand, and bury its own seed-pods. 



One example, although not truly heliotropal, I cannot omit. 

 The Compass-plant of the Texan and other prairies quite invari- 

 ably grows with the edges of its leaves North and South, the 

 faces or surfaces being East and West. This is done in order to 

 expose both surfaces to an equal amount of light, there being in 

 this plant an equal number of sto?nata on both surfaces. 



The trappers use this plant as a compass on dark nights to 

 find their way by, so true is it to its purpose ; and very wonderful 

 is this provision for those who may be even in the daytime lost on 

 the prairies. Mayne Reid, Burton, Lieut. Albert, the Prince of 



