50 PLANT LIFE ON THE FARM. 
concerned, at certain definite places called growing points. 
The new tissues thus formed are at first wholly cellular, 
some of the constituent cells retaining the faculty of sub~ 
division, though sometimes not manifesting it till a later 
period ; while others become modified in various ways as 
growth goes on, forming wood-cells, fibres, epidermis, and 
SO On. 
Form as Dependent on Growth.—If we could sup- 
pose the degree or intensity of growth to be equal on all 
sides, and without impediment or obstacle, the result 
would be a spherical plant; and such plants do exist, 
but, in the great majority of cases, the conditions are 
such that growth is greater in amount in one direction 
than in another; or it may be that while pait remains 
stationary another part grows, the result being a change 
of form. In the case of the main root and stem, the 
principal direction of growth is vertically upwards and 
downwards ; in the case of leaves, the main direction of 
prowth is horizontal, so that while a stem or a root may 
be divided from above downwards into.two nearly equal 
halves, one half the reflex of the other, aleaf must be 
divided horizontally, and the upper surface and the lower 
surface are commonly different. Variations in form are 
dependent not only on variations in the direction of 
growth, but upon the place where growth is taking place, 
and whether it be limited, as in the case of the growing 
points and cambium tissue already referred to, or general 
throughout the mass. 
The form of the plant or of any particular part of it 
will also of necessity vary according as the growth is con- 
tinuous or intermittent, equal or unequal. ‘These are all 
circumstances readily understood, aud they are referred 
to here because they furnish the reasons for the develop- 
ment of bulb and root, as of turnip and mangel as con- 
trasted with that of foliage. In them also must be 
