DAVENPORT: THE ROLE OF WATER IN GROWTH. 75 
of volume which consists of the apposition or intussusception of new 
solid molecules of similar matter (“ welche auf der An- oder Einla- 
gerung neuer fester Molecule gleichartigen Stoffes beruhen”).. 
These definitions include what I regard as only half of the process- 
of growth. 
On the other hand Biitschli has recognized that growth is, in 
part, due to increase of the chylema, and Driesch distinguishes two 
kinds of cell growth : (1) passive growth, due to imbibition of water, 
and (2) active growth, resulting from assimilation. This classifica¬ 
tion agrees with the one I have proposed, but I think the term 
passive growth very inapt, since the imbibition of water is as truly 
an active process as any other vital activity. 
Of the three factors involved in growth — increase of formed 
substance, of plasma, and of chylema — the part played by the last 
seems to me to have been underestimated. Plant physiologists have 
been in the best position to acquire the facts. They have recognized 
in the tip of the plant three growth regions. At the extreme tip of 
the stem (or radicle) is the region of rapid cell division but compar¬ 
atively slow growth; next below is the zone exhibiting the Grand 
Period of growth; and still below is the zone of histological differ¬ 
entiation (Fig. 1) . In the first zone growth of plasma is occurring; 
in the second zone growth of the chylema is chiefly taking place ; in 
the third zone, there is growth of formed substance. The immense 
Fig. 1. Curve of daily growth in length of a disc, originally 1 mm. long, and taken 
immediately behind the vegetation point of a radicle of Phaseolus. It comes to occupy 
in successive days the three zones referred to in the text. From Sachs, Lectures on 
Plant Physiology. 
