57 
Chap. II.] COURSE OF THE SAP. 
their roots would have nothing to imbibe from. 
What faith is the practical man to place in the 
theorist who puts him up to such secrets as 
these? Possibly Liebig may have taken his 
vegetable physiology on trust from others; but 
certainly this is one of a dozen monstrous theo¬ 
ries with which this profound chemist would 
annihilate the very foundations of vegetable 
physiology. “ Mutato nomine, 7 ’ his own words 
to Burdach apply to Liebig: — “ All inquiry is 
arrested by such opinions, when propagated by 
a teacher who enjoys a merited reputation ob¬ 
tained by knowledge and hard labour . 77 
Liebig writes, in reference to ferns, &c.: “They 
resemble in this respect the plants which we 
raise from bulbs and tubers, and which live, 
while young, upon the substances contained in 
their seed, and require no food from the soil 
when their exterior organs of nutrition are 
formed. This class of plants is even at present 
ranked amongst those which do not exhaust the 
soil . 77 According to this, we ought to be able 
to grow our potatoes without any soil at all: but, 
in fact, there is no crop which exhausts the soil 
for itself more than potatoes; there is no crop 
which is more grateful for a change of soil; and 
there is no crop whose growth differs more, 
