Double Adaptations 457 



until direct experimental investigations have 

 been made. 



So it is for instance with the submerged leaves 

 of water-plants. As a rule they are linear, or 

 if compound, are reduced to densely branching 

 filiform threads. Hence we may conclude that 

 this structure is of some use to them. Now 

 two European and some corresponding Amer- 

 ican species of water-parsnip, the Sium lati- 

 folium and Berula angustifolia with their al- 

 lies, are umbellifers, which bear pinnate instead 

 of bi- or tri-pinnate leaves. But the young 

 plants and even the young shoots when devel- 

 oping from the rootstocks under water comply 

 with the above rule, producing very compound, 

 finely and pectinately dissected leaves. From a 

 systematic point of view these leaves indicate 

 the origin of the water-parsnips from ordinary 

 mnbellifers, which generally have bi- and tri- 

 pinnate leaves. 



Similar cases of double adaptation, depend- 

 ent on external conditions at different periods 

 of the evolution of the plant are very numerous. 

 They are most marked among leguminous 

 plants, as shown by the trifoliolate leaves of the 

 thorn-broom and allies, which in the adult state 

 have green twigs destitute of leaves. 



As an additional instance of dimorphism and 

 probable double adaptation to unrecognized ex- 



