102 THE ORIGIN OF MONOCOTYLEDONS [CHAP. 



can often be procured, one half being of the 

 floating type, the other half of the leaf being 

 dissected. I have already observed that if the 

 aerial terrestrial plant with dissected leaves be 

 plunged into water, all the adult leaves at once 

 perish, new ones in adaptation to water soon 

 taking their place ; but all half -developed leaves 

 continue to grow, their new growths, however, 

 conforming to the watery medium. 



We are indebted to Mr W. P. Hiern for 

 demonstrating the truth of one adaptation, 

 for he has shown by mathematical calculations 

 that the best form of the margin of floating 

 leaves for resisting the strains due to running 

 water is circular, or at least the several portions 

 of the margin would be circular arcs, show- 

 ing that nature had adopted what was most 

 serviceable, in this respect, in such leaves as 

 Ranunculus., Nymphceacece, Limnanthemum, and 

 Hydrocharis. Mr Hiern concludes his paper 

 with the following very significant remarks : 



" It is a tenable hypothesis, and by no means 

 improbable, that during much or most of the 

 time, when actual growth is taking place, and 

 when the velocity of the current is subject to 

 many and various vicissitudes, the plant has 

 the power of adapting its growing efforts to 

 the circumstances just necessary for its develop- 

 ment." J 



This hypothesis is completely in accordance 



1 " A Theory of the Forms of Floating Leaves in Certain Plants," 

 Proceedings, Camb. Phil. Soc., fol. xiii. 1872. 



