274 M. WICHURA ON THE WINDING OP LEAVES. 



slight winding occurs here, it must be assumed, at an epoch 

 when the leaves are still quite imperfect, in fact scarcely visible 

 to the naked eye. 



But in all other cases the winding does not commence until 

 the leaf has attained a certain magnitude and maturity. The 

 upper parts, as the older, wind first, the lower then gradually 

 follow. The winding of the older part likewise precedes that of 

 the younger in winding stems. But as in stems the lower parts 

 are older than the upper, while in the leaf the upper parts are 

 the older, the movement advances from below upwards in winding 

 stems, and in the leaf on the contrary from above downwards. 



27. 



In addition to these cases of a gradual origin and progress of 

 the movement, there are some in which the movement begins 

 suddenly and exhibits a more rapid course throughout its whole 

 duration. We then constantly observe that the stage of growth 

 which makes the commencement of the movement, is otherwise 

 of importance in the development of the plant. Thus the rather 

 rapid winding of the legumes of Medicago commences after 

 fertilization has taken place, that of the anthers of many plants 

 after dehiscence, of the petals of Cyclamen europceum, L., and a 

 number of other plants, after the flower has opened, of the 

 flowers of most species of the genus Aristea as soon as they begin 

 to wither, of the awns of Avena fatua and probably all other 

 Grasses with twisted awns, at the beginning of the maturation 

 of the seed, and of the appendages of the carpels of the Gera- 

 niaceaB after their separation from the carpophore. 



t 

 28. 



The movement of winding is in all these cases a rather trans- 

 itory phenomenon compared with the duration of the existence 

 of the plant, but it leaves permanent effects upon the shape of 

 the leaf. When a twisted leaf is unrolled, it returns to its posi- 

 tion as soon as released. It becomes " set " in the movement 

 which has taken place in it, and an unrolling and rolling up of 

 the heliacal winding may be produced in long dead awns of the 

 Grasses or in the fruit-stalks of Mosses, by unequal hygroscopic 

 expansion or contraction of their parts. 



