Nomenclature and Method. 
227 
type of orientation: since an apo-geotropic stem is guided to the 
vertical by the pressure of the falling starch-grains, as a blind man 
gropes his way along a wall by touch. We shall have no further 
occasion to refer to haptotropism, and the question of its right to a 
place among tropic curvatures may be neglected. 
Sachs’ term anisotropic is fortunately no longer in fashion. 
The fact that the root of a bean grows downwards and the stem 
upwards is a case of anisotropism. But it is part of our general 
conception of irritability that different organs may react differently 
to the same stimulus. The words iso- and anisotropic may be 
finally dismissed. 
The same author’s terms orthotropic and plagiotropic are on 
the whole useful and are in general use. An orthotropic part is 
one which is in stabile equilibrium only when its long axis is parallel 
to the direction of the stimulating agent. It may be noted 
however, that if the words geogenic and photogenic were employed 
in the sense already indicated, the terms geotropic and phototropic 
would include the chief cases of orthotropism, but at present 
geotropic is often employed to mean a curvature of any kind 
induced by gravity. Moreover orthotropic has the merit of defining 
the character of the curve without reference to the special 
nature of the stimulus. Pfeffer suggests parallelotropic in 
place of orthotropic : I see no advantage in the change, 1 and it is 
surely rather a cumbrous word, especially when combined, as in 
the expression geo-parallelotropism. Terminology has this, at least, 
in common with wit, that brevity is to be desired in both. 
A plagiotropic organ is one that tends to place itself across the 
line of the stimulus, and what has been said of orthotropism 
applies, mutatis mutandis in this case. It seems to me a mistake 
to use dia-geotropism (or transverse geotropism) for horizontally 
growing organs and klinotropism for cases of obliquity. It is 
convenient to designate by a single term all the secondary roots 
springing from a main root, although they may take up different 
angles with the vertical: I prefer to call them all dia-geotropic. 
Sachs’ assumption of a necessary connection between 
orthotropic sensitiveness and radial structure, and between plagio¬ 
tropic sensitiveness and dorsiventral structure, seems a mistake. 
It drove him to believe in an invisible dorsiventrality in secondary 
roots which are radial and plagiotropic. The matter should be 
1 Pfeffer however uses orthotropism for the straightening of an 
organ by one-sided growth. This use of the term seems 
unfortunate. 
