22 ALISMACEAE [ch. 



Another case, in which the replacement of the inflorescence 

 by vegetative structures has been carried much further, is that 

 oi Caldesia parnassifolia^ (Bassi) Pari., a plant which is somewhat 

 widely distributed in Southern Europe, but does not reach 

 Britain. When it grows in water 30 to 60 cms. deep, the ' in- 

 florescences ' often bear, instead of flowers, vegetative buds 

 about 1 cms. long, which are able to reproduce the plant 

 (Figs. 148 and 149, p. 225). Sometimes these 'turions' as 

 they are called, and also flowers, may occur in the same whorl. 

 Gliick, to whose work on the Alismaceae we owe so much, 

 regards these buds as flower rudiments, which, in consequence 

 of submerged life, have developed in a degenerate vegetative 

 form. This species seems to be losing its power of sexual 

 reproduction, for, even when it bears flowers, they commonly 

 fail to set fertile seed. It affords a good instance of a tendency, 

 common among water plants, to substitute vegetative for 

 sexual reproduction; this characteristic will be discussed more 

 fully in Chapter xvii. 



The range of leaf-form met with amongst the Alismaceae 

 not only in passing from species to species, but also in the same 

 individual under different conditions prompts one to ask 

 which of these divergent types are fundamental and which are 

 derived. Ciliick's study of the family has led him to the conclu- 

 sion that the ribbon form of leaf is primitive, and, on this 

 assumption, he suggests the following scheme, as representing 

 successive phyletic stages which may have occurred in the 

 evolution of the leaves; he admits, however, that the series may 

 conceivably be read in the reverse order. This seriation merely 

 illustrates possible progressive steps and, obviously, does not 

 represent the actual phylogeny of the genera, since examples of 

 Stage I, the most primitive leaf type, and Stage VI, the most 

 highly evolved, are to be found within the limits of the one 

 genus Sagittaria . 



Stage I. Band leaves alone developed, e.g. Sagittaria teres, Watson. 

 Stage 11a. Band leaves extremely important and associated with the 



