ADAPTATIOX 35 



form their seeds in one year, within which their 

 life-histor)' is closed. 



Biennials produce leaves only in the first j'car ; 

 by their aid they la)' up stores of nutriment in the 

 form of tuberous roots, on this food they can exist 

 throuf^h the winter, produce flowers the following 

 summer, perfect their seeds, and then die. 



To this class we owe such useful plants as the 

 carrot, parsnip, beetroot, and man\' others which 

 afford us such nourishing vegetable diet. 



Perennial plants live on for an indefinite number 

 of years, flowering annually, in some cases dying 

 down to the root in autumn, and producing fresh 

 foliage the following year. 



Water plants seldom have a fixed root, but re- 

 main floating, borne up and kept in position b}- the 

 water, their roots being the means b)- which, in 

 conjunction with the lea\es, the)' derive nourish- 

 ment from air and water. It is well worth while 

 to observe the two forms of leaves in the water 

 buttercup. Those on the surface are three-lobed, 

 flat, and niun(l,lhe)' absorb from the air such ga.ses 

 as the plant requires ; while the leaves beneath 

 the surface are di\ idcd into threads so as to offer 



