THE FACTORS OF DISTRIBUTION 91 



The total number of aquatic species of Spermatophytes 

 is not large, and they mostly belong to a few, but very 

 distinct, cohorts. Among Dicotyledons we have the 

 Nvmph&acetz and Batrachian Ranunculi in Ranales, the 

 Podostemacetz among Resales, the Callitrichace& classed 

 with Geraniales, the Mangroves and Trapa among Myrti- 

 florcz, Villarsia and Menyanthes ; in the Gentianacecz 

 and Utricularia in TubiflorcB. Among Monocotyledons 

 we have shore-plants, such as Typha and Sparganium in 

 the Pandanales, the Lemnaceee among Spathiflorce; and 

 the one almost entirely aquatic cohort, the Helobiecs, 

 including Naiadacece, Potamogetonace&, Alismacecs, etc. 

 These varied groups, having many terrestrial allies 

 nearer than they are to one another, and yet possessing 

 many parallel "or representative characters that are 

 obviously adaptations to their acquired habitat, it is 

 clear that the aquatic habit has originated independently 

 in these various cohorts. Though there are many 

 Spermatophytes that vegetate under water, and not a 

 few free-floating rootless forms, many too that ripen 

 their fruits under water, flowering and pollination nearly 

 always take place above or on the surface. The pollen 

 and pollination generally suggest that these submerged 

 plants are the modified descendants of land-plants. We 

 cannot, in fact, point with confidence to a single 

 Phanerogam as primitively aquatic. At the same time, 

 the extreme modification of such entire orders as 

 Naiadacea. Ceratophyllacece, and Podostemacecs, which 

 renders their systematic position doubtful, points, in 

 such cases, to an extremely remote aquatic ancestry. 

 Water has such high specific and latent heat that it is 

 much slower than air to change its temperature. Aquatic 

 plants are, therefore, little liable to have their growth 

 checked by changes of temperature, or to be prevented 

 from spreading over wide areas within the same zone 

 of temperature. Almost all aquatics are consequently 

 perennial, and they exhibit a general tendency to- 

 vegetative growth and multiplication, as opposed to 

 sexual reproduction. Whilst in tropical waters this 

 vegetative growth takes place uninterruptedly all the 

 year round, in temperate latitudes the danger of destruc- 

 tion by the formation of ice at the surface leads to 

 various forms of hibernation, such as the sinking of 

 detached winter-buds. 



