170 LEAVES FROM THE BOOK OF NATUKK. 



fluids of a certain consistency, and thus select those sub- 

 stances which are best adapted to the growth and welfare 

 of the plant. The finer, suitable material is taken in, 

 the coarser rejected. Eepeated, careful experiments have 

 proved this beyond doubt. A grain of wheat and a pea, 

 raised in the same soil, and under absolutely the same 

 circumstances, draw entirely different substances from the 

 earth. The wheat consumes all the silica or flinty matter, 

 that water can absorb, while the pea takes up no flint, 

 consuming, on the other hand, whatever lime or calcareous 

 matter the water of the soil may contain. 



Thus the roots of a plant pump up nearly all the nu- 

 triment that is required and at least ninety-nine per cent 

 of all the water which the plant needs, the only other part 

 needed being brought by the vapors of the atmosphere 

 and absorbed through the humus. They perform this duty 

 with a vigor little suspected by the inattentive ; but if 

 we cut a vine and fasten a bladder to the wound at the 

 time when the sap is rising, it will in a short time be 

 filled and finally burst ; and it has been stated that the 

 root of an elm-tree, which was by accident badly wounded, 

 poured forth, in a few hours, several gallons of water. 



Not all roots, however, have to perform this difficult 

 and responsible task of extracting food from the earth 

 around them ; those of aquatic plants draw it directly 

 from the water itself, as in our common duckweed, where 

 each little leaf has its own tiny root, a single fibre, which 

 hangs from the lower surface. In the mangrove, on the 

 contrary, they form a kind of enormous network in the 

 water, which intercepts all solid matter, that floats down 



