PHYSIOLOGY. 
BOOK H. 
into a kind of disk. From this expansion the roots are 
emitted, and penetrate the interior of the branch whereon 
the seed of the miseltoe is fixed : its stem takes the directions 
above mentioned with reference to the centre of the branch on 
■which it is fixed, and not with reference to the earth; so that, 
with regard to the latter, it is sometimes ascending, some- 
times descending, sometimes horizontaL The same pheno- 
mena occur if the germination takes place upon dead wood 
or inorganic substances : a number of seeds were glued to the 
surface of a cannon ball ; all the radicles were directed 
towards the centre of the ball. Hence it is obvious that the 
tendency of the miseltoe is not towards the surface of its 
nutrition, but it obeys the attraction of the body upon which 
it grows. The miseltoe, which does not grow on the earth, 
obeys the attraction of any other body ; while those plants 
which naturally grow in the earth obey no other attraction 
than that of the earth. Parasitical fungi, those which con- 
stitute mouldiness ; aquatics, which originate on stones, — all 
grow perpendicular to the body that produces them, and will 
therefore be placed in all kinds of positions with respect to the 
earth. 
The tendency downwards of the roots, and upwards of the 
stem, is chiefly observable in the ascending and descending 
caudex ; that is to say, in the axis of the vegetable considered 
as a whole. The lateral emissions of this axis always deviate 
from its direction in a greater or less degree : we know that 
the roots produced by the tap root, and the branches which 
proceed from the side of the principal stem, scarcely ever 
take a direction absolutely vertical. This is probably due to 
several causes, one of which is undoubtedly the general ten- 
dency of all the parts of plants to take a direction perpen- 
dicular to the plane of the body on which they grow. The 
branches of trees are to those which produce them what the 
miseltoe is to the branch on which it vegetates : but, as there 
is a double attraction operating upon all branches, — that is to 
say, an attraction towards the stem and an attraction upwards, 
in consequence of the general law to which they all submit, — 
it results that a middle direction is taken, and, instead of one 
