6o6 



ECOLOGY 



thin, rosette leaves are more divided than are the small, 1 hick, stem leaves. 

 In Cytisus scoparius the three-lobed lovi^er leaves are followed by simple 

 (or more correctly, one-lobed) leaves, greatly reduced in size, and in 

 the Japan ivy the lower leaves are three-parted, while the upper leaves 

 are simple (though with similar palmate venation) and often as large 

 as or even larger than those below (figs. 887, 888). In Leonurus Cardiaca 

 the intermediate stem leaves are more divided than are those below or 



Figs. 889-894. — Leaf variation in the motherwort {Leonurus Cardiaca) ; 889, a 

 basal rosette leaf; 890, one of the more apical rosette leaves; 891, a leaf from the lower 

 part of a stem ; 892, 893, leaves from the upper part of a stem ; 894, a leaf from the floral 

 region; note that lobation and leaf size increase for a time, culminating in 891, and reach- 

 ing a minimum in 894. 



above (figs. 889-894), and in Sassafras almost any leaf may be entire or 

 variously lobed, apparently without relation to transpiration or nutrition, 

 or even to the phenomena of rejuvenescence. One of the most extraor- 

 dinary cases of leaf dimorphism is exhibited by the staghorn fern 

 (Platycerium) , in which there are erect leaves that bear the sporangia, 

 and sterile leaves that adhere closely to the ground and keep the substra- 

 tum constantly moist. The sterile leaves appear to react to contact and 

 gravity rather than to light, ultimately forming an overlapping series, 

 the lowermost of which gradually become transformed into humus. 



The advantages of leaf variation in land plants. — In so far as the 

 basal or lower stem leaves of any plant are large and thin, they favor 



