88 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



THE OEIGIN AND STEUCTUEE OF AQUATIC 

 FLOWEEING PLANTS. 



By the Eev Prof. G. Henslow, M.A., F.L.S., V.M.H., &c. 



[Read April 11, 1911.] 



General Inferences. — There are good reasons for believing all exist- 

 ing aquatic flowering plants to have descended from terrestrial species. 

 First, because botany has of late years greatly supported the hypo- 

 thesis that Dicotyledons are descended from terrestrial Gymno- 

 sperms* ; secondly, that Monocotyledons were derived from aquatic 

 Dicotyledons ; and lastly, because most of the genera and species of the 

 families to which aquatic plants belong are, as a rule, terrestrial. 

 Thus the Primrose family is terrestrial, but one genus, Hottonia, 

 is aquatic. Buttercups and other genera of the same family are 

 land plants ; but the many forms of the Water Crowfoot are aquatic. 

 Earely a whole family is aquatic, as the Water-lilies (Nympkaeaceae) ; 

 but this family is allied to Ranunculaceae through the genus Cahomha 

 with submerged dissected leaves. ' 



Of the six external factors which chiefly affect the life of plants — 

 viz. water, temperature, soil, air, light, and other organisms — the 

 difference in degree of the amounts of water and heat are by far the 

 most important in affecting the forms and internal structures of plants. 

 Thus a hot, moist, tropical forest is totally different from a dry thorn- 

 forest or savannah, though the air, light, and soil may be more or less 

 alike. Similarly, in temperate regions, an amphibious plant or an 

 aquatic one with submerged leaves when growing on dry ground is very 

 different from the same species growing in its normal habitat. 



Turning to the causes of structural changes, observations and 

 experiments have conclusively proved that the external conditions are 

 the actual causes of the new structures which appear when the seeds 

 of a species accustomed to one kind of locality grow up in a very 

 different environment, as in changing from dry to wet, from hot to 

 cold, conditions, or vice versa, &c. The changes are due to a " re- 

 sponsive " power residing in the life of the plant, which directs the 

 forces acting in and on the molecules of matter, which build up tissues 

 different from those of the parent plant in order to put the offspring in 

 adaptation to its new conditions of life. 



Such is the real basis of the whole evolutionary process. 



How did terrestrial plants become aquatic ? It is probable that they 

 passed through the intermediate stage of becoming marsh plants. 

 Thus Ranunculus Lingua and R. Flammula, our two spear^^orts, have 

 narrow, pointed leaves, as if they had grown thickly on probably very 



* " The Xerophytic Characters of Coal-plants," Quart. Journal Geol. Soc, 

 vol. Ixiii. (1907) p. 282. 



