100 THE ORIGIN OF MONOCOTYLEDONS [CHAP. 



Similarly with Sagittaria sagittcefolia, there 

 are ribbon-like phyllodes in deep water, the 

 plant when bearing them alone being known 

 as the form vallisneriifolia. Then follow con- 

 secutively, as the surface of the water is 

 approached, the spatulate and hastate forms. 

 Finally, there occurs the sagittate type, but 

 this is now aerial. The sagittate-peltate form 

 is attained by Caladium and Alocasia, but the 

 orbicular form is not reached in these genera. 

 It is seen, however, in Hydrocharis, resembling 

 the dicotyledonous Limnanthemum, etc. Con- 

 versely, the ribbon-like form so characteristic of 

 monocotyledons is imitated by the leaves of 

 Lobelia Dortmanni, Limosetta aquatica, Hippuris, 

 Callitriche, Littorella lacustris, etc. 1 From the 

 above considerations one arrives at the con- 

 clusion that those terrestrial monocotyledons 

 which still retain a linear form (e.g., grasses 

 and sedges), or ensiform (Iris), or other similar 

 type of leaf, or rather phyllode, may be 

 regarded as representing the ancestral sub- 

 merged ribbon - like forms ; and those mono- 

 cotyledons with distinct blades of a lanceolate 

 or oval shape (Convallarid) represent the float- 

 ing type, like Potamogeton natans; while the 

 sagittate form, so characteristic of the Aroidece, 



1 Such leaves perhaps resemhle those of the earliest mono- 

 cotyledons, being developed from a primitive cotyledonary type. 

 It is noticeable that the cotyledons of many of the allied order 

 Onagracece have only a single midrib. 



