work of Vochting, "Ueber Organbildung im
Pflanzenreich", I. Bonn, 1888; also "Bot. Zeit." 1906, page 101; cf.
Goebel, "Experimentelle Morphologie", Leipzig and Berlin, 1908, Section
V, Polaritat.) It is found, for example, that roots are formed from the
base of a detached piece of stem and shoots from the apex. Within
the limits of this essay it is impossible to go into this difficult
question; it is, however, important from the point of view of our
general survey to emphasise the fact that the physiological distinctions
between base and apex of pieces of stem are only of a quantitative
kind, that is, they consist in the inhibition of certain phenomena or
in favouring them. As a matter of fact roots may be produced from the
apices of willows and cuttings of other plants; the distinction is thus
obliterated under the influence of environment. The fixed polarity of
cuttings from full grown stems cannot be destroyed; it is the expression
of previous development. Vochting speaks of polarity as a fixed
inherited character. This is an unconvincing conclusion, as nothing can
be deduced from our present knowledge as to the causes which led up to
polarity. We know that the fertilised egg, like the embryo, is fixed at
one end by which it hangs freely in the embryo-sac and afterwards in
the endosperm. From the first, therefore, the two ends have different
natures, and these are revealed in the differentiation into root-apex
and stem-apex. A definite direction in the flow of food-substances
is correlated with this arrangement, and this eventually leads to a
polarity in the tissues. This view requires experimental proof, which in
the case of the egg-cells of flowering plants hardly appears possible;
but it derives considerable support from the fact that in herbaceous
plants, e.g. Sempervivum (Klebs, "Variationen der Bluten", "Jahrb. Wiss.
Bot." 1905, page 260.), rosettes or flower-shoots are formed in response
to external conditions at the base, in the middle, or at the apex of the
stem, so that polarity as it occurs under normal conditions cannot be
the result of unalterable hereditary factors. On the other hand, the
lower plants should furnish decisive evidence on this question, and
the experiments of Stahl, Winkler, Kniep, and others indicate the right
method of attacking the problem.

The relation of leaf-form to environment has often been investigated and
is well known. The leaves of bog and water plants (Cf.Goebel, loc.
cit.