208 MorcGan: Hyporuesis OF FORMATIVE STUFFS 
of the experiments with the isolated leaves of the foliaceous liver- 
worts can be best explained without assuming polarity, which in 
turn he here supposes to be associated with the absence of fibro- 
vascular bundles in the leaves. He ignores the well-known fact 
that in many of the algae, which are extremely simple in struc- 
ture, the polarity is as strongly expressed as in the regeneration 
of the higher plants. Moreover, the unicellular animals and plants 
show well-marked polarity. At other times Goebel speaks of 
certain growing regions —apical buds or cells—as drawing on 
the general supply of certain substances, and by continually ex- 
hausting these inhibiting the growth of other buds less advanced 
or less fortunately situated. He does not, I believe, sufficiently 
discriminate between this possibility, and the other view of the 
direction of the flow itself determining the development of certain 
parts. 
I shall try to keep apart the idea that the presence of some 
particular substance may be the immediate cause of the growth of 
a part, and the idea of the flow of particular substances in a given 
direction as the immediate cause of growth. Moreover, it will be 
well to discriminate between the two possible causes of the flow. 
The polarity itself is sometimes hinted at by Goebel as the cause 
that determines the direction of the flow. On the other hand we 
may suppose that the flow is always towards the region where less 
of the substance exists and from the region where more is present. 
The theory of mobile stuffs was first invented to explain the 
phenomenon that we call polarity. It is especially this part of 
the theory that I contest. I do not deny that the presence of 
certain substances in a piece may determine its reaction, neither do 
I question that a flow of certain substances in definite directions 
in the plant takes place, but I suspect the real cause of the flow 
in a given direction is dependent on the presence of certain organs, 
where the substances are being used or changed into other sub- 
stances. Let us take a specific case and make definite applications 
of these different points of view. If a piece of the stem of a 
willow-plant is cut off and suspended in a moist atmosphere (its 
leaves having been previously removed) with its apical end upward, 
it will be found that the buds near the upper end of the piece 
begin first to grow, and grow more rapidly than those below them. 
