84 



CHAPTER VII 

 THE LIFE-HISTORY OF CERATOPHTLLUM 



EACH of those aquatic families whose life-histories we 

 have hitherto considered, contains a considerable num- 

 ber of species, representing, in the case of the Lemnaceae, three 

 genera, while, in the case of the other groups discussed, the 

 number is much higher, as many as fourteen genera being in- 

 cluded, for instance, in the Hydrocharitaceae. The family 

 Ceratophyllaceae, the subject of the present chapter, offers a 

 marked contrast on this point, since it includes only a single 

 genus, containing three species, or, on other interpretations, one 

 alone 1 . Ceratophyllum, the Horn wort, is extremely isolated in 

 its structure and habits, so much so that there has been, at 

 various times, the widest diversity of opinion as to the posi- 

 tion which should be assigned to the family; the plant, from 

 its taxonomic wanderings, has been opprobriously styled " a 

 vegetable vagabond." The question of its affinities will be 

 discussed in Chapter xxv. 



In the genus Ceratophyllum the aquatic habit seems to have 

 reached its ultimate expression. The plant not only lives entirely 

 submerged throughout its vegetative life, but even its stigmas 

 do not reach the surface, and the pollen is conveyed to them by 

 the water 2 . The Hornwort is monoecious, the male flowers con- 

 sisting of a group of stamens enclosed in a perianth of about 

 a dozen members (p in Fig. 54 5). These stamens, when the 

 flower is mature, become detached the terminal expansion of 

 the connective acting as a float 3 and rise to the surface of the 

 water. They then dehisce and the pollen, having a specific 

 gravity very slightly higher than that of water, sinks gently, 



iSchleiden, M. J. (1837). 



2 Delpino, F. and Ascherson, P. (1871). 3 Ludwig, F. (i'88i). 



