WATER-PLANTS 55 



multiply by vegetative means, and how difficult and 

 uncertain seed - formation is with them. Again, the 

 annual, represented in winter by seed only, is clearly 

 more suited to life on land than in water, for it is here 

 that winter conditions are more pronounced. Among 

 the alleged aquatic annuals found in Great Britain may 

 be mentioned the duckweeds {Lemna), ZannichelUa, and 

 awlwort (Subularia). 



The Origin of Aquatics. 



It is generally assumed that the first forms of life 

 originated in water, and that the earhest plants on 

 the globe were aquatics. By a gradual process of 

 modification some forms became fitted to live on 

 land, and these, emerging from the water, gradually 

 established themselves on dry soil, and became the 

 forerunners of all subsequent land-plants. That was a 

 long time ago, when the world was stUl yoimg, and the 

 first stratified rocks were being laid down under water. 

 But from that day to this, through the countless ages 

 of geologic time, plants have been changing and modifying, 

 the better equipped races ever driving the weaker ones 

 out of the fair and pleasant places on the soU. The 

 flowering seed-plant is a late arrival on the scene, the 

 highest and the most successful expression of natural 

 adaptation in the long line of descent of land-forms. 

 Certainly no plant could have evolved the seed-habit 

 while it was submerged in water ; the flower is a useful 

 structure only in the air. From this we conclude that 

 every seed-bearing plant that hves in the water to-day 

 is not a primitive aquatic, but has been derived from 

 ancestors that once lived on the land. In the struggle 

 for existence, certain plants were bound to be driven 

 off their habitats by stronger and better equipped 

 competitors. Those among them that were able to 

 adapt themselves to the conditions of other environ- 

 ments, lived on ; some took to the water, others to the 

 hills, where competition was less keen. These were 

 preserved from extinction ; those which could not so 

 adapt themselves perished. Our modern flowering 

 aquatics therefore are, in every case, the descendants of 

 plants which were thus worsted in the struggle for existence 



