Variation as an Organic Function 185 
usually associated with the self-preservative or reproductive instincts. 
At a higher stage it includes the complex external economic machinery 
of insect colonies which often incorporates within itself the physi¬ 
ological reactions of different species of organism. On a different 
line of evolution is the progressively increased capacity for functional 
adaptation during somatic life which begins with the power of 
external movement, thus making the organism less dependent upon 
the primary environment, and finds its highest expression in 
intelligent behaviour. 
All these various examples are essentially so many different ways 
in which simple types of organism have been rendered more efficient, 
either by an increased power of selection with respect to their re¬ 
actions with the outside world,- or by the superaddition of some 
external economic machinery to the internal physiological mechanism 
previously evolved. 
MERISTEMATIC TISSUES AND PROTEIN 
ISO-ELECTRIC POINTS 
By W. H. PEARSALL and J. H. PRIESTLEY 
Introduction 
I n some recent studies of the development of cork within the 
plant an attempt has been made to give a causal interpretation 
of the appearance of this tissue. There is one aspect of the 
whole problem which is neglected in both these communications, 
but which is really fundamental in dealing with the development 
of meristematic tissues. This aspect it is desired to consider in the 
present note. 
Cork usually arises in fully differentiated and vacuolated paren¬ 
chymatous tissue as the result of the appearance of a meristematic 
phellogen. Both this meristem and the cells formed from it are 
often completely filled with protoplasm and always much less highly 
vacuolated than most tissue cells. The transformation of tissues from 
a vacuolated, non-dividing condition into non-vacuolated, or only 
slightly vacuolated meristematic cells is characteristic not only of 
phellogen formation but also of the extension of vascular cambium 
across the ground tissue bordering the vascular bundles. In either 
