M. WICHURA ON THE WINDING OF LEAVES. 267 



the Monocotyledons occur wound much more frequently than 

 the stem-leaves of the Dicotyledons, on account of their structure 

 being peculiarly suited for the movement of winding. 



9. 



It is only when we come to the families of plants that it be- 

 comes possible to sketch out general characteristics of the natural 

 divisions, in reference to the movement of winding. There are 

 families in which I have not hitherto detected a single example 

 of a winding movement, as for instance the Umbelliferae and 

 Cruciferae ; families in which the winding is confined to particular 

 genera or species, as in the Composite and Papilionaceae ; and, 

 finally, families in which the curvature of the leaves is typical, 

 as in the Ebenaceae, Apocynaceae and Asclepiadaceae. 



The same holds good of the genera, with the distinction that 

 a complete agreement of the species included in them is much 

 more frequent than in the families. 



In general, the behaviour of the individuals belonging to the 

 same species is identical. Deviations are rare, and seem only 

 to arise when the shape of the leaves has undergone a material 

 change through the influence of the station. Thus I found in a 

 hot-house a specimen of Hyacinthus orientalis, raised in rich 

 earth, in which the stem-leaves had attained the unusual length 

 of 2 \ feet, and were twisted, while under ordinary circumstances 

 the leaves of this plant do not wind. 



10. 



Finally, peculiarities of the soil and climate also have to be 

 mentioned among the causes of the diffusion of the phenomenon, 

 since these sometimes give a particular character to the habit of 

 the plant, and especially to the prevailing form of the stem- 

 leaves. Thus, tropical America is remarkable for an abundance 

 of plants with broad leaves with reticular venation, in which the 

 here incompatible movement of winding withdraws from notice, 

 and, considered in relation to other regional floras, becomes rare. 

 On the other hand, the floras of New Holland and the Cape of 

 Good Hope contain principally plants with narrow, parallel- 

 nerved stem-leaves, and it is to be regarded as a consequence of 

 this, that Dicotyledonous plants with winding stem-leaves occur 



