282 POWERS OF SELECTION. [BOOK n. 



that in. Cycads gum is formed at the expense of the starch of 

 the stem, and that such a change is effected by the action of 

 the free oxalic acid secreted in the leaves. 



We are, therefore, to understand hereafter that gum is a 

 form of the nutritive matter of plants; that, instead of being 

 the result of vegetable digestion, it is a principle created by 

 nature for their crude food; that one at least, if not the 

 principal of the functional purposes for which starch is 

 universally dispersed through the tissue of plants, is in order 

 that it may be everywhere ready for conversion into gum ; 

 and finally that it is in the form of gum that starch passes 

 through the sides of the tissue in which its granules were 

 originally generated. 



Fixed as plants are to the soil, deprived of volition, and 

 incapable of removing their highly absorbent roots from what 

 is hurtful to them, except with extreme slowness, it appears 

 scarcely probable that they should have any power of select- 

 ing their food ; on the contrary, the facility with which they 

 are poisoned would seem to show their helplessness in this 

 respect. But, if roots are made to grow in coloured infusions, 

 it is said that they take up only the colourless parts, leaving 

 the coloured behind ; and we know that if an apple tree is 

 planted in a piece of ground in which another apple tree has 

 been growing many years, the new plant will languish and 

 become unhealthy, whatever quantity of manure, that is of 

 new food, may be offered to its roots. This last fact is 

 accounted for upon the supposition that the soil contains 

 some peculiar principles which are necessary to the health of 

 an apple tree, and that the old tree having selected for its 

 own consumption all that the soil contained, has left none 

 behind it for the new comer. It has been, however, demon- 

 strated by Daubeny, that plants have, to a certain extent, a 

 power of selection by their roots. He found that when barley 

 was watered with distilled water, containing in every two 

 gallons two ounces of nitrate of strontian, not a trace of that 

 earth could be detected in the ashes of the plants ; and when 

 Lotus tetragonolobus was treated in a similar manner, except 

 that only two ounces of nitrate of strontian were dissolved in 



