Farmer —On Isoetes lacustris , L. 
43 
The cells lying between the apical merismatic layer of the 
stem and the xylem of the vascular bundle evince, as has been 
already said, a general convergence towards the apex, and it is 
at the periphery of this mass that the cambium is first differ- 
entiated, and it is gradually pushed outwards below by the 
products of its own activity. Hence, as in the higher plants, 
radial divisions are essential to admit of the attainment of the 
increased circumference which the zone occupies. There are 
two methods whereby these radial divisions are effected : (i) 
Either, a cell of the cambium divides into two equal halves 
X 
A 
\ 
V 
ABC 
Fig. i. Diagram of transverse section of cambial cells. 
in the radial direction, and then each of the two sister-cells 
divides henceforth tangentially in the usual manner to form the 
secondary tissue (see Fig. A below) : (2) Or, a cambial cell 
divides into unequal sister-cells by a wall which cuts one of 
the radial walls and the inner tangential wall, and then the 
first succeeding divisions are not quite tangential, but are 
curved to meet the rounded cell-wall ; but finally the r esult 
is the same as in the first case, though the products can be 
more easily traced, at any rate inwards, on account of the 
pointed end by which such a cell-row is always characterised 
(Figs. B, C, above). These two modes of division are however 
