THE ORIGIN AND STRUCTURE OF AQUATIC FLOWERING PLANTS. 93 



identical with the flat, hnear blades of some aquatic Dicotyledons, as 

 Littorella, &c., already mentioned. In these it will be found that the 

 veins running parallel to one another branch at the base of the blade 

 from the central midrib, whereas in the linear leaves of Monocotyledons 

 they all enter the sheath separately, and then run parallel from the 

 base to the apex. 



On approaching the surface of the water, the parallel veins widen 

 out, and so an elliptical blade is formed. Again, by a lateral down- 

 ward growth, the blade becomes spear-head shaped. This is fol- 

 lowed by an arrow-head shaped blade. This may be the final stage, as 

 in the Arrow-head (Sagittaria). The two basal points may, however, 

 give way to a rounded form, the blade becoming cordate, as in the 

 Water-lily. If the petiole grow up into the air, the lower parts of 

 the blade may more or less grow together and become peltate, being 

 sagittate as well in Caladiu7n, but circular in Nelumbium and our 

 moisture-loving Hydrocotyle. 



These stages can be well seen and compared with one another in 

 Victoria and Sagittaria. 



The Degeneracy of the Reproductive Organs. — The flowers of 

 Water-lilies, Buck-bean, Villarsia, and Water- violet are conspicuous 

 and attract insects, which visit them for honey. If, however, flowers 

 are neglected by their insect friend's they become inconspicuous by 

 degeneration, and, especially if growing in water, they may become still 

 more degraded in consequence. This may be seen in the Mare's-tail, 

 Water-Milfoil, Water- Starwort, and the Sea Grasswrack. In such 

 the calyx and corolla may vanish and the stamens and carpels be 

 reduced to one each. ^ 



But further, in Dicotyledons the degeneracy may attack the 

 minutest microscopical structures of the pollen formation as well as of 

 the embryo sac and the embryo within it. This last may be arrested 

 in growth at the pro-embryonic state, consisting of a minute globular 

 mass of cells without any differentiation at all. 



An ordinary terrestrial dicotyledonous embryo is seen in germi- 

 nating mustard and cress. It has two primordial leaves or cotyledons, 

 with or without a stem below called the hypocotyl. The terminal bud 

 between the cotyledons is the plumule, and the stem is continuous 

 with the root under the ground. 



The first stage of the embryo is the same in all plants and consists 

 of the globular mass of cells, without any sign of the above-mentioned 

 parts. This is the condition at which water sometimes arrests it in 

 the seed, as in Ranunculus Ficaria. Consequently such seeds take a 

 long time to germinate. 



In Water-lihes the formation of the two cotyledons has been 

 observed. A crescent-shaped ridge is formed on the globular ''pro- 

 embryo." This ridge bears one of the cotyledons. The other cotyledon 



* I have described them more fully, with figures, in my Heredity of Acquired 

 Characters in Plants (Murray) and in A Theoretical Origin of Endogens from 

 Exogens through Self-Adaptation to an Aquatic Habit. Journ. Lin. Soc. Bot. 

 xxix. p. 485. See ako Annals of Botany (1911). 



